Arrange washed chilies, tomates and onion on a tray, preferably in an arrangement easy to clean yet not dependent on tinfoil which will likely glaze to the vegetables. Then again, since you're removing the skins, maybe this is not such a big deal.
Ensure even the small ingredients have enough room to get roasted. Roast ingredients under broiler, turning frequently until evenly charred. Some ingredients will char faster than others. Tomatoes will lose this race, being as they are mostly water.
Remove skin from chilies and tomoatoes, placing the former between two damp towels or thicknesses of paper towel to steam any attached skin off.
Chop all ingredients (including cilantro), removing stem and seeds from chiles.
Mix ingredients and let rest for a few hours to let flavors develop. Add salt and pepper to taste.