This sofrito recipe is a pretty standard version of this Spanish sauce, consisting of onions, peppers, tomatoes, and spices.
In a large pan over medium heat, fry onions in oil until translucent.
Add pepper and garlic to onions. Briefly fry, stirring to incorporate.
Add tomatoes and spices to pan. Cook, stirring frequently, until tomatoes are soft and sauce has thickened slightly.
Remove bay leaf before use.
Made this with a red bell pepper and canned Roma tomatoes. It ended up only making ~3 cups, but I think that can be attributed to me botching the conversion between fresh plum tomatoes and canned Romas. I can't really comment on how tasty it is (no salt so didn't taste it plain), but it seems to work well in carne molido.