Step 1Blacken the peppers: Place the peppers over direct high flame of a gas burner, over very hot grill, or under broiler close to heat, turning as needed until blackened on all sides, 3 to 4 minutes. Remove the peppers to a bowl, cover with a dish towel, and set aside to steam until the peppers are cool and their skins are loosened. Peel the peppers and discard stem end and seeds, reserving any liquid that collected in the bowl. You should have about 2 cups of roasted pepper. Strain the pepper liquid, if necessary, to remove any burnt bits or seeds.
Step 2Finely mince the onion. (If not using sweet onion, “de-flame” it: Place the onion in a sieve and rinse several times, then place it in bowl of water and stir. If water turns milky, repeat the process. Soak for 30 minutes, rinse and dry well between paper or dish towels.)
Step 3Toast the garlic, if you like, in a small skillet over medium heat until blackened in spots. Drop the garlic through feed tube of food processor with motor running. Add the peppers, reserved pepper liquid, and salt to the work bowl and process to a puree. With motor running, add the oil through the feed tube to emulsify. Scrape the mixture into a bowl, and stir in the onion. The sauce will keep, at room temperature, for several hours, and up to a week refrigerated.