1. Place the beef in an extra-deep 12 inch skillet over medium heat. Begin browning the beef, stirring often to break up the meat. Meanwhile, peel and coarsely chop the onion, adding it to the skillet as you chop. Cook, stirring often, until all the meat is crumbled and cooked. Serve at once or cool and store for future use.
2. Reduce the heat to medium-low. Add the taco seasoning and tomato soup, and stir until well blended. Add the cheese, and stir until it melts, about 30 seconds. Remove the skillet from the heat, and stir in the sour cream until well combined. Serve at once, or store for future use (see below).
Makes about 5 cups
NOTE: This filling can be refrigerated for up to 2 days or frozen for 2 to 3 months. Refrigerate or freeze the filling in 1-cup portions in microwave-safe containers. To defrost, remove the container lid and microwave on defrost setting or defrost in refrigerator overnight. DO NOT use ground beef that has been previously frozen--you aren't supposed to freeze it twice.
3. Use this filling in these recipes: