What gives Indian curries the sweet and creamy taste and fragrance?

Well I do the following for punjabi gravies-

1. I add poppy seeds, cashew and melon seeds in milk, soak them for some time and then grind these in blender. This gives it a creamy texture and that white shade in the red gravy
2. I cook the onions properly, as well cooked onions give a sweetened flavor.
3. To some paneer gravies I add little sugar. I know it sounds weird but it does give gravy a different and tasty flavour.
4. Adding cream from also gives it a sweet flavor.
5. For that spices flavor, I will suggest to roast the spices dry first and then grind them, this will give a flavor as well as fragrance to the gravy.

Though I am not much aware of dhaba flavours but making punjabi gravys by these points will surely give better flavors than restaurants

My first rule: what you want to be the "strongest" taste should go first with oil. That way the oil is infused with the and everything that goes next get coated and give it's own taste only after biting on it.

So I will put oil and then mix in my spices, In curry I think the strongest taste of "sweet" comes from star anise punched up with ginger. So make sure you get that in good quality.

Onions - if you use ones, use white sweet/sugar ones. Yellow or red can be more oniony than sweet.

Garlic - I just went over two heads of garlic that smelled and tasted like matchstick head. VERY strong sulfuric taste without anything close to what is usually associated with garlic.

I use coconut milk or cream - some of them are sweet, some just "coconutty". If I have the latter I mix one or two spoons of brown sugar in it before adding everything to the dish.

Spices - becasue I really like coriander and turmeric I make my own curry mix. With a lot of the sweet spices and rest (like mustard, pepper) added later in the cooking process. AS i usually cook rice with chilli I omit in the spices.

Two ideas to bring the sweet

1. Toast nuts. I like slivered almonds in curry. If you pan toast them they get sweeter. You can also toast unsweetened coconut shreds and they get a fine toasty sweetness - a good mix with coconut milk / cream curries. I have toasted coconut shreds on a pan in the toaster. Watch the coconut shreds closely and get them out when tan because they burn quick.

2. Fenugreek. A little goes a long way but fenugreek seeds taste like maple syrup. Crush them up before adding.

I would like to suggest that the sweet fragrance you mention is probably primarily coriander seed powder. Coriander powder is, to my knowledge, usually the major component of garam masala, as well as in other commonly used ground masalas like sambar powder. Coriander powder is used in volume, not only because of its magical citrus-honey scent but also its mildness and lack of bitterness compared to other spices. Both garam masala and coriander powder are added to those creamy gravy curries. Other sweet-smelling spices such as cinnamon (which may be the secondary source of sweet aromas, or the main source in curries that don't have coriander), cardamom and nutmeg are also usually included in garam masala.

The actual sweetness and creaminess of such curries meanwhile, usually comes from fried onions and either cashews and/or dairy products. Instead of (or in addition to) onion, ground coconut may be used. White poppy seeds and/or watermelon seeds are also used in some creamy sauces.

Here is a sort of general recipe for a rich creamy gravy curry sauce like the one for paneer butter masala:

• 1 tbsp ghee (or, since I'm vegan I use 2 tsp coconut oil and 1 tsp olive oil)
• 1/4 tsp cumin seeds (optional)
• 1 clove, crushed (optional)
• seeds of 1 cardamom, crushed (optional)
• 1 Indian bay leaf (optional)
• 1 green chilli, chopped
• 1 red onion, roughly chopped
• large handful cashew nuts (about 10-12)
• 1 tsp fresh ginger, roughly chopped
• 1 tsp fresh garlic, roughly chopped
• 1/2-1 tomato, chopped (about 2-3 tbsp chopped)
• salt to taste
• 1 tsp coriander powder
• 1/4 tsp garam masala
• red chilli powder to taste (optional)
• 1/3 cup thick curd/yogurt or cream (since I'm vegan I use cashew almond curd)
• 1/2 tsp dried fenugreek leaves (kasuri methi)

1. Fry the cumin, bay leaf, clove and cardamom for a few seconds
2. Add onion and chilli and cook until the onion is turning brown
3. Add cashews, ginger and garlic and cook for a minute
4. Add the tomato with salt and cook until the tomato turns totally mushy
5. Add coriander, garam masala and chilli powder if wanted. Take off the heat and allow to cool somewhat, at least so it's not steaming hot. Remove the bay leaf.
6. Grind the mixture to a paste and return the paste to the heat until it starts to simmer.
7. Add curd or cream and kasuri methi

Sauce is ready: add paneer, cooked veggies or whatever.

I make garam masala similar to Richa Hingle's recipe which has:

• 1/2 cup (40 g) whole coriander seeds
• 1/4 cup (25 g) cumin seeds
• 6 to 8 (6 to 8 ) 2-inch cinnamon sticks
• 8 to 10 black cardamoms
• 2 tablespoons green cardamom pods
• 2 tablespoons cloves
• 1 tablespoons (1 tablespoons) black peppercorns
• 10 to 12 Indian bay leaves
• 1 nutmeg optional

P.S. I can heartily recommend Dassana Amit's recipe blog. You may find this dhaba style recipe of hers helps you figure out the magic formula.