Time indexing image collections in GEE

I’ve been trying to list a ee.ImageCollection() in GEE but by using time indexing and I came up with this solution:

var geometry = ee.Geometry.Polygon([[-86.036445612425396,
11.175779794553685 ],
                                [ -86.036445612425396, 9.362885788210679 ],
                                [ -83.883633979893077, 9.362885788210679 ],
                                [ -83.883633979893077, 11.175779794553685 ], 
                                [ -86.036445612425396, 11.175779794553685 ]]);

var start_date = '1979-01-01' 
var end_date = '2019-12-31' 
var Landsat = ee.ImageCollection('LANDSAT/LE07/C01/T1_TOA')
                    .filterBounds(geometry)
                    .filterDate(start_date,end_date);
var listOfImages = Landsat.toList(Landsat.size())

By doing this, I can index each image like this:

var firstImage = listOfImages.get(0)
Map.addLayer(ee.Image(firstImage))

However, this method indexes one scene of the collection each time. I mean, firstImage is the Landsat scene LANDSAT/LE07/C01/T1_TOA/LE07_015052_20000221.

I would like to know how to mosaic an image collection and index them by time. For example, getting the whole mosaic as a result of running this instruction: Map.addLayer(Landsat.median().select('B4')), in general, being able to get Map.addLayer(Landsat.get(nth-image).select('B4')).