Mash an avocado, then stir in lime juice, yellow onion and salt.
Microwave a flour tortilla for 10 seconds to make it more pliable. Spread 2 tablespoons of the avocado mixture evenly on top. Layer on 1/4 cup of the beans, spreading it evenly. Roll up the tortilla. Trim away 1/2 inch from each end, then slice the tortilla into 1-inch-wide pieces. (For easier cutting, you can first wrap the tortilla in plastic and freeze it for 5 minutes.) Repeat with 3 more tortillas and the remaining fillings.