Finely chop the onions, peppers, and garlic. Take care when chopping the hot peppers so as not to burn your fingers (disposable gloves are recommended).

Place a large skillet over medium-high heat and add vegetable oil.

Heat the oil until it shimmers and then add the onions, peppers, and garlic. Cook vegetables, stirring regularly, until the onions begin to brown, about 8 to 10 minutes.

When onions are ready, add tomatoes, vinegar, chili powder, and salt. Stir to combine and cook until tomatoes soften and the liquid has reduced.

Remove pan from heat and let the sauce cool for a few minutes.

Carefully transfer the sauce into a blender or food processor and puree.

Pour sauce through a sieve to remove any remaining chunks and funnel into the waiting jars.

Let jars cool and refrigerate.