Step 1

Line heavy large skillet with foil; heat over medium heat. Place onion and garlic in skillet; cook until onion softens and blackens in spots, turning often with tongs, about 15 minutes. Transfer onion and garlic to medium bowl. Peel garlic.

Step 2

Place same foil-lined skillet over medium-high heat. Working in 2 batches, cook tomatoes in skillet until tender and skins blister and blacken, turning occasionally, about 20 minutes. Transfer to large bowl; cool and peel. Working in batches, puree tomatoes, onion, garlic, and chiles in blender until smooth.

Step 3

Heat oil in heavy large deep skillet over medium-high heat. Carefully add tomato puree to skillet (mixture will bubble vigorously). Stir in salt. Reduce heat; simmer gently until sauce thickens slightly, stirring often, about 15 minutes.