1Heat oil in skillet over medium high heat. Stir in onions; cook about 2 to 3 minutes or until soft. Stir in refried beans. Cook for 2 to 3minutes until hot, stirring occasionally. Let cool for 5 to 10 minutes, to keep tostadas from getting too soft.
2To assemble tostadas: Spread 2 tablespoons refried bean mixture on each shell; top with 2 tablespoons nopales, 1 rounded tablespoon tomatoes, 1 scant tablespoon red onion, 2 slices avocado, and 1 tablespoon queso cotija. Sprinkle with cilantro.