Heat oven to 350 degrees F. Place tortillas on work surface and spread each with 1/4 cup salsa. Divide onion among tortillas and sprinkle each with 1/3 cup cheese.
Place tortillas on baking sheet and bake at 350 degrees F for 17 minutes or until cheese is melted and tortillas are crisp. Remove from oven and top each with 1 tablespoon guacamole. Serve immediately.