Pick over and rinse 1 lb. pinquito (small pink) beans. Place in a 6-quart pot and cover with water. Bring to a boil, cover, take off heat and let sit 1 hour.
Drain beans. Cover with fresh water, bring to a boil, lower heat to a simmer, and cook until tender, anywhere from 40 to 90 minutes depending on the freshness of the beans. Drain, reserving beans and 1 cup cooking liquid.
In a 5-quart pot over medium-high heat, cook 4 slices (about 1/4 lb.) bacon, chopped. When bacon is crisp, remove all but 1 teaspoon of fat from the pan. Add 2 chopped onions (12 oz. total) and 2 cloves minced garlic to the pan. Cook, stirring, until onion just starts to brown. Add 1/2 cup tomato purée, 1/2 cup red chile sauce or red enchilada sauce, 1 tablespoon brown sugar, 1 teaspoon dry mustard, and the pinquito beans and reserved liquid. Salt to taste. Bring to a boil, stirring often, then lower heat and simmer for about 10 minutes to blend flavors.