Heres an easy one and its versatile, its called white sauce and once you have the base you can add several different things to give it other uses
The base for 1 sauce:
2 tbsp flour
2 tbsp butter
1 cup milk
Salt and pepper to taste
Melt the butter in a pan and add the flour, cook it for around 2 minutes stirring constantly to cook out the flour taste. Add the milk in thirds stirring constantly until thickened and add the next third of milk until all milk is added. Cook to desired thickess add salt ad pepper to taste
This can be used on potatoes or on noodles add cheese to make a taco sauce or thicken to make a gravy