Start by making the marinade. In a food processor or blender, combine the coarsely chopped red onion, garlic cloves, adobo sauce, ancho chile powder (or soaked dried ancho chiles), olive oil (or rice bran oil), ground cumin, dried oregano, salt, and freshly ground black pepper. Blend everything until you have a smooth mixture. If needed, add water to reach 1 cup of liquid.