Step 1

Special Equipment: broiler; sauté pan.

Step 2

Position an oven rack in the upper third of the oven and preheat the broiler to high. Line a medium-sized rimmed baking sheet with parchment paper. Halve the tomatoes and remove the seeds. Place cut side down on the baking sheet. Slice the onions into 1/4-thick rounds and arrange in a single layer on the baking sheet. Broil until the tomatoes and onions are soft and the tomato skins and edges of the onions are charred, about 10 to 12 minutes.

Step 3

While tomatoes and onions are broiling, in a medium skillet set over medium-high heat, warm 2 tablespoons of the oil. Swirl to coat the bottom of the pan. Add the garlic cloves and use tongs to turn to brown all sides, about 3 minutes. Add both chiles and stir frequently to toast them, about 2 minutes. They will darken but remove from the heat before they blacken.

Step 4

Transfer the chiles and garlic and any oil remaining in the pan to the blender. Add the vinegar, water, and salt and let the chiles soften in the liquid for 5 minutes. Add the broiled tomatoes and onions along with the remaining oil. Blend on speed 3 until the sauce is emulsified and creamy orange in color. The sauce will be thick. Transfer to bottles or a glass quart-size jar with a tight-fitting lid. Refrigerate until ready to serve. The sauce will keep for up to 3 weeks.