Is there an antonym (a complementary antonym) for "spicy" or "hot" regarding food (I DO NOT mean "seasoned", but "hot")?

The antonyms for spicy are bland, and tasteless which means eating something that is unflavored. If the OP wants to eat something tasty, they'll have to go down a different route.

• unspicy

While the entrees hardly blew us away (the garlic tilapia and angel hair was bland, and the spicy Romano pizza, though decent, was unspicy and unremarkable), we were happy enough just making a meal of the steaming, fresh-from-the-oven bread... Indianapolis Monthly, Nov 2005

• zero scoville

The levels of hotness are measured in multiples of 100 units, from the completely harmless Bell pepper at zero Scoville units to the Habanero pepper at 300,000 Scoville units.

An observation

It seems a lot of unnecessary attention is focused on the falafel recipe and dish itself, when the OP just wanted a word or unequivocal expression meaning absolutely no spice or not ‘spicy’ hot. However, something needs to be pointed out, even if the OP ordered the plainest falafel dish on the menu, it would still have cooked onions (or shallots). Raw onion is pungent and sharp tasting but cooked onions are softer, sweeter-tasting and more easily digested.

I have a relative who recently discovered she cannot eat anything that contains onion, garlic, leek, shallot etc. We went to a London restaurant, and when it was time to order, she asked for the fish of the day but added: “Can I have it without any onion or garlic?” She politely emphasised that she couldn't eat onion, the waiter was only too happy to oblige. Similarly, the OP can ask the server: “Can I have [dish] without any spice, please?”

Contrary to what you may think, the single word that people actually use to distinguish from spicy is:

mild.

This is often used to see how spicy you want something:

"How do you want your spaghetti sauce: hot, medium, or mild?"

That's the continuum. If you are talking about other dimensions of seasoning, like say turmeric or horseradish or bay leaf or rosemary, range is 'heavily seasoned' to 'bland' (having little to no taste at all).

Your observation that one person's mild is another's too spicy is simply a matter of psychology. The word 'mild' means 'not spicy', the opposite of spicy.

If you want to make sure that something has no capsicum style seasoning at all but not bland, which is to say if you want to say something that is out of the ordinary (which this is) , you have to use more than one word.

no red pepper at all

not hot at all

totally bland

or whatever extent you care for. And for your given sentences:

Falafel stand: Would you like your falafel with X?

Me: No thanks, I don't want anything spicy.

It seems here though it's not a word choice problem but how food prep and requests work. When you buy some prepared food, there's the basics and then some extras. At some places, they ask you about every single extra ingredient. At some places, they put most of it together as the basics, then you can add a couple of things like, hot sauce or pickles or whatever. If they ask if you want hot sauce and you don't like hot things, then just say 'no thanks'. If it's still too hot, then maybe it's not mild enough for you.

In my opinion, it can’t be done.

You point out one of the problems in your question: “mild” can be a relative term. Sometimes (as in mass-market salsa in the USA), you can be sure that “mild” means zero hot pepper or peppers bred to have no heat. That is likely attributable to the Pace salsa company which innovated by breeding jalapeños that have no heat at all, so their mild and medium salsas would taste exactly the same as the hot salsa without the heat. But outside of a grocery store, things get messy very fast. So problem 1 is that one word never means the same thing to everyone.

Problem 2 is that many people have no association between the flavor groups of hot peppers (Capsaicin); onion, garlic, and shallot (allium); and ginger, mustard, horseradish, and wasabi. Those are often considered completely different flavor categories. It is common (at least in the US) to call something with a lot of mustard/wasabi “hot”, even for someone who tastes them as totally different flavors, but if you say “not hot” at a place that has both hot peppers and ginger or wasabi available (e.g., Asian restaurants) they will almost certainly interpret it as “no hot pepper, wasabi is fine”.

Problem 3 as hinted at in my clarifications is that these kinds of words and phrases are highly regional. If you told us exactly where in what country you were planning to use a suggested phrase or sentence, we might be able to suggest something that works maybe 80% of the time. But travel even 50-100 miles in many parts of the world and all bets are off.

Problem 4 is something not specific to your question. The unfortunate truth is that restaurant servers often want to just get their job done and move on to the next customer. They will sometimes tell a customer whatever they want to hear and deliver whatever food they have available in the shortest amount of time so they can make some other, easier customer, happy. Anyone who is vegetarian, vegan, or has an allergy or other dietary restriction can tell you this. I myself am celiac, so I have a very bad reaction to gluten, but I often can’t get a solid answer from servers or other restaurant staff. If the menu is marked, I can deal with it, but mainly I only eat at home and a few places I trust (found through trial and error). There’s certainly no phrase that I can say that will make restaurant staff stop what they are doing, completely understand my requirements, know all the ingredients of all the food served, and take the time to explain my options to me. Instead a common response is “it only has a little bit” or “I don’t know”.

My advice to you:

• Carry a snack with you when you are going out for a long time or to a restaurant so if you can’t find or get tolerable food you don’t go hungry.
• Learn what kinds of foods are usually safe and gravitate toward those. For me, if I can order a salad I’m usually ok. For you, maybe it’s fruit or French fries or something. When picking a restaurant, say something about “I’d prefer a place that has X” so you know you can order at least one thing.
• Find places that either already make things that work for you or where they understand your order after you explain it to them.
• Eat at home a lot.

Good luck!

I came up with an answer. Peppermint. Peppermint oil and leaves have a cooling effect. With much thought, this is the closest thing I could come up with as an opposite.

There is a flavour of Doritos tortilla chips which uses no chili powder and is called "cool original". "Cool" seems to be a sensible antonym to "spicy hot" as it is consistent with the usage relating to physical heat.