Melt butter on the stove, add in flour so it thickens Use a wisk and add in milk and continue cooking over a medium-low heat for 2-3 minutes or until it's bubbly and thick.
Add in cheese and stir until melted. Take off the heat. Layer some of your potatoes in a baking dish, or corningware. Sprinkle with a bit of garlic salt.
Pour over 1/4 of the cheese. Repeat layer of potatoes and cheese until your pan if full. Bake for about 60 minutes or until top is golden brown and potatoes are tender.