Here it is, and apologies for the lack of specific quantities but it came from a friend that seldom measures.

Saute a couple of chopped onions in a good blob of butter, add the flour and milk to make a white sauce, then dump in your grated cheeses. Marilyn used the large bag of Kraft Mozzarella and cheddar mix from the grocery store to save time, but said she'd rather have had some gruyere in there. Salt and pepper to taste and a whole sprig of thyme. Toss in the thinly sliced potatoes and a scant cup of water.

Whack the whole thing into the crock pot, on high for 4 hours. It was delicious.

When I do get specific amounts, shall let you know.