Spicy Black Bean Filling

1 lb. dried black beans
1/2 onion, diced
2 jalapeños, minced
1 serrano, minced
1 tbsp comino
salt & pepper to taste

Sort through the beans and place them in a large stockpot. Cover with water (at least 2" over the beans), put the lid on and bring to a boil. Boil for 30 minutes. Carefully drain the water, add fresh water to cover along with a dribble of oil. Cook until tender.
Meanwhile saute the onion, jalapeños, and serrano until cooked through. Spoon the cooked beans into the skillet with the sauteed onion and peppers, mash little by little until all of the beans are incorporated and smooth. Add a little of the cooking liquid if it seems dry. Season with comino, salt, and pepper. Fill tamales, adding cheese if desired.