Rinse beans in filtered water and put them in a large pot with 6 cups of water. Let them cook for about two hours or until the beans are soft.
Meanwhile In a bowl mix the masa harina with a tablespoon of oil, the mint, the serrano chile, 1/4 teaspoon salt and 3/4 cup of water. Add water gradually until a manageable dough is formed. Make the dumplings.
In a separate pan add the tablespoon of oil and the chopped onions with half a teaspoon of salt. Sautee the onion for about five to seven minutes.
Add the cooked onions and all the dumplings to the beans. Cover and let them cook for 10 minutes. Taste for seasoning before serving, adjust if necessary.
Notes
If using canned beans, first sautee the onion with one tablespoon of oil and a pinch of salt over medium-high heat.Then add the two cans of beans with 3 cups of water, smash the beans a bit to develop a bit of more flavor in the soup.Add more liquid if it is a bit thick, add the dumplings and let them cook for ten minutes.