The solution for saving potatoes that have been freshly sliced from darkening prematurely is to slice and drop them into a bowl of water with some acidity to it. My “go to” choices are adding a little white wine vinegar to the water or concentrated lemon juice. In a gallon of water, it only takes a teaspoon or so, won’t negatively affect the flavor of the dish at all. So, cut and drop the potato slices into the water and acidic solution for at least 5-10 minutes, drain and add the butter, cheese and spices and bake off.