Preheat the broiler with the rack adjusted at the highest setting. Cover a baking sheet with foil and place on the tomatoes, chiles, white onion and garlic cloves. Broil for about 10 minutes, flipping the vegetables over halfway through, until charred and the tomatoes are mushy and beginning to release their juices. Remove from the heat.
Alternatively, you can toast the vegetables on a comal over medium heat, flipping them over every 4 to 5 minutes, until charred.
Place the chiles in a plastic bag, or in a bowl and cover tightly with plastic. Allow the chiles sweat and cool. Then stem, peel and seed the chiles under a thin stream of water. Peel the garlic cloves.
Coarsely chop the roasted tomatoes (don’t remove the skin, seeds or core), onion and cleaned chiles and place in a bowl, along with the garlic, and mix with a fork. Alternatively, you can mash them all together in a molcajete. Add the lemon juice and salt to taste, and serve. The salsa will keep for 5 days in the refrigerator.