Preheat oven to 350°.
Place tortillas, wrapped in foil, in the oven and heat for 10 minutes.
Meanwhile, sauté onions with cooking oil in a medium-sized saucepan until translucent. Stir in refried beans and cook until it reaches the desired temperature.
Remove tortillas from the oven. Portion the beans and cheese evenly in the center of each tortilla.
To assemble, first fold one half of the tortilla until it covers the filling. Take the adjacent sides and fold them over the first fold. Holding the three folded sides taught, and wrap the fourth corner tightly over the other three.
Serve with guacamole, sour cream, and salsa as desired.