½ cup shredded Colby-Jack cheese, or more to taste
Instructions
Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, 15 to 20 minutes. Drain.
Return potatoes to the pot and mash until smooth. Add sour cream, cream cheese, margarine, and milk; beat until light and fluffy. Add in garlic powder, salt, and pepper. Transfer to a 9x13-inch casserole dish.
Bake in the preheated oven until heated through, 30 to 40 minutes. Sprinkle Colby-Jack cheese over top and bake until cheese is melted, about 5 more minutes.