Melt butter in a small saucepan over medium low heat. Add flour, garlic salt, black pepper and cayenne pepper and stir until smooth and bubbly.
Add milk and cream slowly, stir until smooth. Bring mixture to a boil, remove from heat and cover to let white sauce thicken while preparing potatoes and onions.
Peel and slice potatoes and onions in a food processor or mandolin, and place in a small greased cassarole (8"x8" or 9"x9") in an even layer.
Pour white sauce over potatoes and onions.
Cover pan with aluminum foil and bake at 375° for 40 minutes. Remove foil and continue to bake for 30 minutes more, until potatoes are cooked and top is nicely browned.