In a pan, sauté onion and bell pepper in oil for 2-3 minutes. Combine black beans, chopped spinach, taco seasoning, and salt. Cook for another 1-2 minutes. Set the mixture aside.
In a large pan or griddle, drizzle a thin layer of oil. Spread the cheese(optional), beans mixture, and again cheese on the tortillas. Wrap the tortilla into half around the filling.