Rinse beans and put them in a large pot with 6 cups of water. Cook for about 2 hours or until soft. (If using canned beans, skip this step.)
In a bowl, mix the masa harina with oil, chopped mint, minced serrano, salt, and 3/4 cup of warm water. Add water gradually until you get a playdough consistency. Roll into small balls (about 1 inch) and press your thumb into the center of each one.
In a separate pan, heat oil and sauté the chopped onion with salt for 5 to 7 minutes until soft and translucent.
Add the sautéed onion to the beans. If using canned beans, add them now with their liquid plus extra broth or water.
Drop all the dumplings into the pot at the same time. Cover and cook for 10 to 15 minutes until the dumplings float and are cooked through.
Taste for seasoning, adjust salt, and serve.
Notes
For canned beans: use 2 cans (30 oz total) of black beans. Sauté the onion, add beans with liquid, add 2 cups of broth or water, bring to a simmer, then add dumplings.
The masa should feel like playdough. If crumbly, add a little more water. If sticky, add more masa harina.
Make all dumplings before adding them to the pot so they cook evenly.
Serve with sliced avocado, lime, and a drizzle of crema.