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.
Cook for 2-3 minutes per side on medium-high heat, or until the outside is crispy and the cheese has completely melted. Repeat until all tortillas are filled and all fillings are used.