Step 1In a large saucepan, place the tomatillos and jalapenos and add enough water to cover. Bring the water to a boil over high heat, then reduce the heat to a simmer and cook the tomatillos until they soften and turn a yellowish-green, about 10 to 15 minutes (timing will vary depending on the size). Drain the water and place the tomatillos and jalapenos in a blender with the onion. Blend to a rough puree (pulse the blender quickly, it should only take a few seconds).
Step 2Heat the oil in a large saucepan over medium heat until hot. Carefully add the tomatillo paste (it will splatter). Fry the paste over medium heat, stirring with a wooden spoon, until the sauce thickens and darkens a little, about 5 minutes. Taste the sauce and adjust the seasoning and acidity with a little salt and sugar (to balance the acidity) as desired. Remove from heat and set aside. This makes about 2 cups sauce. Cool before serving.
Step 1In a large bowl, soak the beans in at least 6 cups of water for 8 hours, or preferably overnight
Step 2Drain the beans and place them in a ceramic Mexican bean pot or a heavy casserole with 2 quarts of water, the quartered onion and the epazote. Bring the liquid to a boil over high heat. Lower the heat and cook the beans, covered, at a gentle simmer until tender, about 2 hours (time will vary according to the age and size of the beans). The beans should be soft but retain their texture.
Step 3When the beans are done, strain them from the liquid (save the liquid) using a slotted spoon, picking out as much of the onion and epazote as you can find, and discarding it. Place 1 cup beans and 1 cup of their cooking liquid into a blender and puree until smooth.
Step 4In a large, heavy-bottom soup pot heated over medium-high heat until hot, add the oil. Stir in most of the chopped onion (reserve a little to use as a garnish) and garlic. Add the nopalitos, frying until they are heated through, a few minutes more. Stir in the drained beans and the puree from the blender, stirring for 2 to 3 minutes to marry the flavors. Measure the remaining cooking liquid from the beans, adding enough water to come to 6 cups; stir the liquid in with the beans and nopalitos.
Step 5Bring the liquid to a slow boil, stirring from time to time. Reduce the heat to a slow simmer and cook for 20 minutes to thicken the soup; the nopalitos will cook through and lose their bright color. Taste and add salt as desired.
Step 6This makes almost 3 quarts soup. Serve the soup in bowls, garnished with the sauce and a little of the reserved chopped onion.