Soak beans overnight in cold water. Drain.

Heat oil over medium heat in a large, heavy-bottomed pot. Add onion, pepper, and garlic and sweat until onions are translucent, 4–6 minutes. Add squash and sauté until slightly browned, 4–5 minutes.

Add sofrito, tomato sauce, baking soda, seasoning mix, and beans. Add enough water to cover the beans by 1 inch. Bring to a boil. Reduce heat to a simmer, cover, and cook until beans are soft, about an hour. Stir every 15 minutes or so to ensure the beans don’t scorch. As you cook, skim the foam from the surface. When the beans are fully cooked, season to taste with salt.