You can pretty much add this over anything. Do it several times and become familiar with it and use it. I am just going to use skim milk and Kraft singles as that's what most people have. I would stick to the skim milk just to keep it healthy, but def get as fancy as you want with the cheeses. Anything works.
2 C skim milk
2 leeks (whites only), cut up finely
3 TBSP butter or marge
3 TBSP flour
3 slices Kraft singles
1 tsp dried mustard (only if you have... def not necessary)
Dash of nutmeg (again, only if you have... def not necessary)
Set sauce pan to medium heat and add butter and leeks. Stir until leeks become tender (about 12-16 minutes). Add the flour (making a rouix... see Macaroni and Cheese recipe for rouix discription) and stir for another 5-8 minutes until the mixture is slightly brown. Whisk in the milk very slowly and reduce heat to low. Let simmer for 5 more minutes then add the cheese slices slowly until the dissolve (this should take about another 8 minutes). Add the dried spices and let simmer until the rest of your meal is cooked. Smother everything with this ish - it's like crack!