10 dry Choricero peppers, or substitute Guajillo peppers, available at most Mexican and specialty food markets
3 tablespoons olive oil
4 ounces Serrano ham or bacon, diced small
8 cups thinly sliced yellow onion
3 tablespoons kosher salt
4 cloves garlic, thinly sliced
1 teaspoon pimentón or any quality unsmoked
1 cup canned tomatoes
Toast the peppers in a dry, hot pan on both sides until just pliable, about 30 seconds. Remove the stems and tear open peppers to remove and discard seeds. Place seeded peppers in boiling hot water, using a weight to keep them from floating. Let steep until they are plump and you can scrape the rehydrated flesh away from the skin with a spoon. Discard the skin. Reserve the pepper pulp as well as the water.
Meanwhile, in a 3-quart pot, heat olive oil gently and add ham or bacon. Cook for 5 minutes and add the onions and garlic, coating everything in oil. Stir for 5 minutes uncovered. Add salt and cook covered over low heat until onions are sweet and wilted. Uncover and continue to cook, stirring until onions are very sweet and uniformly golden but not browned; you are looking to melt the onions. Add the reserved pepper pulp and pimentón. Cook 5 minutes. Increase heat to medium high and add the tomatoes, crushing them with your hand as you add. Cook until the tomatoes have lost their acidity. Remove from heat and let cool for at least 20 minutes.
If using Choricero peppers add a pinch of Cayenne powder for punch. If Guajillo are used, they should have plenty of punch. Transfer to
a blender or food processor in batches and purée while warm but not hot. If needed, add reserved pepper soaking water back to mixture.