Basic Cheese Sauce

Very simple. So far, works great with cheddar or parmesan. No reason other cheeses wouldn't work. No need to use a melting-friendly cheese (neither good cheddar nor parmesan melt particularly well). Basically just a bechamel with cheese added at the end.

30g butter
30g flour
salt & pepper
500ml whole milk
100g cheese

melt butter
stir in flour and cook a bit over low heat
[optional] heat up the milk in the microwave until hot
whisk hot milk into butter & flour, off of heat
put back on heat and stir until boiling and thickened
take off heat, add cheese, stir gently until smooth and creamy

add the salt & pepper at any step in which you remember to do so, but keep in mind some cheeses can be quite salty so it's fine to save it for the end
can put it on low heat during the cheese melting stage if needed, but don't boil it once the cheese has gone in
shredding the cheese works better than dicing