Heat 2 tablespoons of oil in a large, heavy soup pot or casserole over medium-high heat. Add the onion and cook for 5 to 6 minutes until it has completely softened, the edges are golden brown, and there is a toasted and sweet aroma wafting from the pot. Add the garlic clove and and cook for another minute until the garlic is fragrant and has colored. Stir in the tomatoes and 1/2 teaspoon salt and cook for 5 minutes until the tomatoes have cooked down to a soft, thick paste.
Add the beans along with 1 cup of their broth, as well as 4 cups of the chicken or vegetable broth. Bring to a boil, reduce heat to medium, cover partially and simmer for 10 minutes. The beans should be completely soft and the broth thick and soupy.
Meanwhile, prepare the masa for the dumplings. In a medium bowl, combine the corn masa flour with the water and 1/4 teaspoon salt. Knead together with your hands. The dough will be very coarse and seem dry. Add the remaining 2 tablespoons of oil, queso fresco, cilantro, and mint, and mix together until the dough is very soft and homogenous, about 1 minute. Set aside.
Working in batches, puree the pinto bean soup in a blender until completely smooth. Pour back into the soup pot and whisk or stir in the remaining 4 cups chicken or vegetable broth. Set over medium heat and bring to a simmer. Reduce heat to low.
Begin forming the dumplings. Scoop up enough masa to make 1-inch balls, roll them between your hands (moisten your hands with water if they stick and, one-by-one, gently drop them into the soup. Once all the masa balls have been shaped and added to the soup, gently stir with a wooden spoon to make sure none stick to the bottom. Cover the pot partially with a lid and let the soup simmer gently for 15 to 20 more minutes until the masa dumplings are cooked through. They will thicken the soup as they simmer.
Taste the soup for salt and add more if need be. Serve hot, garnishing each bowl with a spoonful of fresh Mexican cream if desired. You may also sprinkle on some sliced scallions, mint, cilantro, and crushed chiltepín chiles or chiles de árbol for a punch of heat.
Note: If you are lucky enough to live next to a tortilleria or store that sells fresh corn masa, already mixed, go for it! You will need about 3/4 pound. Just mix it with the queso fresco, oil, mint, cilantro, and salt. If you can’t get ahold of fresh masa, rest assured that the corn masa made with masa harina for tamales or tortillas will still be excellent. If you have a choice between corn masa flour for tamales or for tortillas (they are two different products; masa harina for tamales will say so on the package), go for the tamal mix for these dumplings. The masa harina for tortillas is finer than the flour for tamales, but it will still work.