Heat the oil (or use water) in a frying pan over medium heat. Add onions and cook until soft (about 5 minutes). Stir in salt, then set pan aside. Across the middle of each tortilla, sprinkle 2 tablespoons cooked onion and shredded cheese. Roll to enclose. Place tortillas, seam side down, in a greased, shallow 9 x 13 inch baking pan.