4 tbsp of butter
1 small onion, small diced chunks
1/4 cup of all purpose flour
1 quart milk
2 cups cheese (can be a blend of mozzarella and cheddar, or simply use marbled cheese)

Melt the butter over low to medium heat till it’s all melted and immediately add the onions and turn the heat up a bit to medium. Cook the onions till they are translucent, and then add the flour. Stir the mixture to cook the flour into the butter and onions. When all of the flour is wet and cooked a bit, add the milk. Cook this mixture for about 8-10 minutes till the starch from the flour thickens the milk. When the sauce is sufficiently thick enough to coat a metal spoon, use a whisk in the pot to ensure all of the flour lumps are broken down before adding your cheese and folding it in till it’s all melted. Remove from heat and let stand a few minutes, or add the sauce to your casserole or entrée dish immediately.

Fairly easy, no?