Roast the tomatoes, habaneros and sliced onion on a rimmed baking sheet about 4 inches below a hot broiler for 5 or 6 minutes, until everything is blackened and blistered. Remove the onion and habanero, then flip the tomatoes over and roast them for another 5 or 6 minutes, until they are blackened and soft. Cool until handleable and, if you wish, pull off the blackened skin from the tomatoes.
Roughly chop the habanero(s) and scoop them into a blender or food processor, followed by the tomatoes. Pulse to roughly chop everything, then run the machine until you have what I’d call a very coarse puree—not too chunky, certainly not smooth. Scrape the mixture into a salsa dish. Chop the onion into small bits and stir them in, along with the cilantro and a good dose of salt—it usually takes nearly a teaspoon. If the salsa is too thick to easily spoon, stir in a tablespoon or two of water.