Here's the link for the recipe: https://www.tasteofhome.com/recipes/simple-au-gratin-potatoes/
I made the following changes: used shallots instead of onions and I minced them and sauteed them in the butter with some dried parsley and a splash of white wine before adding the flour. I added the mild very slowly to ensure the cheese sauce was smooth. I used yukon gold potatoes. Just before I put it in the oven, I sprinkled it with paprika. So delicious!