Heat oven to 400℉.
In a large bowl or hotel pan, combine the butter, soup, sour cream, onion, salt, and pepper.
Stir in 3/4 lb each Colby-Jack and cheddar cheeses (reserving 1 cup each for topping).
Stir in the potatoes.
Transfer mixture to a one hotel pan or divide between two (13 x 9) bake dishes (sprayed with cooking spray).
Top with the reserved 1 cup each Colby-Jack and cheddar cheeses.
Bake at 400℉ for 80 minutes or until potatoes are cooked soft in the center.
If dividing this recipe between tow 13x9-in bake dishes, the cooking time will shorten to about an hour.
If the top starts to get too browned while cooking, lay a large piece of foil loosely overtop while potatoes finish cooking.
Calories: 403kcal | Carbohydrates: 27g | Protein: 15g | Fat: 26g | Saturated Fat: 15g | Monounsaturated Fat: 2g | Cholesterol: 90mg | Sodium: 368mg | Potassium: 151mg | Fiber: 1g | Sugar: 4g