To make the sauce, heat a cast iron skillet or comal over medium heat. Place the jalapeño, 2 cloves garlic, and quartered onion on the hot comal. Cook the vegetables, turning frequently, until they are nicely charred on all sides (the garlic will cook the fastest, so remove as soon as it is done and keep cooking the other vegetables), about 15 minutes.
Meanwhile, cover the 1/2 pound tomatillos with water in a medium saucepan and bring to a boil. Lower heat to a simmer and cook until tomatillos are soft, but not falling apart, and have lost their bright, green color, about 5 minutes. Drain and place in a blender.
Add the charred vegetables to the blender along with the 1/2 cup chopped cilantro, teaspoon salt, and 1 cup of the vegetable broth and purée until smooth. Transfer to a bowl and set aside.
Wipe out the skillet or comal you used to char the vegetables and return to the heat. Add the 1/4 cup pumpkin seeds, 1/4 cup peanuts, and 2 tablespoons sesame seeds and toast, stirring occasionally until fragrant and browned.
Transfer toasted nuts to the blender and add remaining 2 cups vegetable broth. Purée until smooth.
In a deep frying pan heat the 2 tablespoons of oil over medium heat. Add the tomatillo mixture and the nut mixture (be careful it will spatter). Cook, stirring frequently until sauce is thickened, about 10 minutes. Taste and add more salt as needed. Remove from heat and cover until ready to use.
To make the enchiladas, heat broiler to high. Combine 1 can drained black beans, 1/2 teaspoon cumin, and 1/4 teaspoon oregano in a bowl and mash together until combined, but not completely smooth.
Pour 1/2 cup vegetable oil in a medium skillet and heat over medium heat.
Once the oil is hot, place tortillas, one at a time, in the oil and fry on one side until the edges are crisp, but still pliable, about 30 seconds, flip and repeat on the other side. Remove from oil and dip each side in the Pipían sauce.
Fill each tortilla with about 2 tablespoons of the black beans then roll up and place seam-side down in a baking dish. Repeat with the remaining tortillas and filling.
Ladle about 1/2 cup of sauce over the top of the tortillas and place under the broiler until toasted and brown, about 3-4 minutes. Remove and serve immediately with your favorite toppings.