3 Tbsp flour
3 Tbsp butter (or oil)
1+1/2 cups milk (or half and half, or light cream)
8oz cheese (at least 8oz, I like swiss and cheddar)
Salt to taste
Melt butter, add flour and stir until smooth. On low, add milk in small parts, stir until smooth (a whisk works best). After milk is incorporated, cook on med-high until it starts to boil, continue whisking and cook for 5 mins. Add cheese and stir yet again until smooth. Salt to taste.