In a large skillet, brown ground beef and onions until no longer pink. Drain grease and return to pan.
Add taco seasoning and cook according to package directions.
In a small pan, heat refried beans until warm.
Preheat a large non stick frying pan (this will be used to cook the tortillas once they are filled).
Place ¼ cup of shredded lettuce and a handful of diced tomatoes on top of each tortilla followed by a hard tostada shell.
Add sour cream to each hard tostada shell then add a spoonful of refried beans, cheese and taco meat on top.
Place a ¼ piece of the 5th tortilla in the center on top of the taco meat and start folding the edges of the bottom tortilla towards the middle making sure all the ingredients are covered with the tortilla.
Place folded tortilla into the frying pan and cook until both sides are lightly browned.
Serve with guacamole, queso, sour cream and rice, if desired.