1. Whisk the flour into the skim milk in a small saucepan, then place it over medium-low heat.
2. Add the shredded cheddar cheese, margarine and salt and stir often with a spoon until the cheese begins to melt. Reduce the heat to low and simmer for 30 minutes. Use a whisk to stir the sauce every couple of minutes so that it becomes smooth and thick.
Use 2-9" tins to make 5 serving portions to freeze or one large cake tin for party size serving.
3. While the sauce thickens, prepare the macaroni by dumping 3/4 cup uncooked elbow macaroni into rapidly boiling water. Boil for 8 minutes or until tender, and then strain. You should have about 7 cups of cooked macaroni.
4. When the cheese sauce has simmered for 30 minutes, pour the pasta into a medium bowl. Gently stir in the cheese sauce, and then pour the mixture into a loaf pan or casserole dish. Cover and freeze.
5. When you are ready to dive into your macaroni & cheese, preheat the oven to 350 degrees F. Bake the frozen macaroni for 50 to 53 minutes, or until the cheese begins to brown slightly.
Tidbits: While I prefer the oven method of heating up the macaroni & cheese (since it adds a bit of browning and a slightly crunchy texture), you can also use your microwave oven to prepare the dish in a fraction of the time. Just be sure you freeze the macaroni & cheese in a microwave-safe casserole dish following the recipe above. When you are ready to heat up the frozen cheesy awesomeness, simply cook it uncovered on high for 8 minutes, and then stir. Continue cooking on high for 2 to 3 more minutes or until the macaroni & cheese is hot
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