Heat oil in a large nonstick skillet over medium heat. Add onion, oregano, garlic, and jalapeño; cook 8 minutes, stirring frequently. Stir in sherry, soy sauce, beans, and seitan; bring to a boil. Cook 7 minutes or until liquid almost evaporates. Sprinkle with black pepper.
Prepare taco shells according to package directions. Spoon about 1/3 cup bean mixture into each shell; top each taco with about 2 1/2 tablespoons lettuce and about 2 1/2 tablespoons Avocado Salsa.