Boil and drain pasta shells. Mix mashed potatoes, garlic salt, onion salt, dried minced onions and 4oz. cheddar cheese. I have used this recipe both ways â heated mashed potatoes and cold mashed potatoes, they both work well. I preferred using the heated mashed potatoes . Using a small melon ball scoop, gourmet decorator or an Easy Accent decorator or even a small spoon fill the stuffed shells with the potato mixture. Place stuffed shells in a greased 9 x 13 pan. Top with remaining cheese, green onions and Potato Topping (I placed 3 very small dabs of butter on top of cheese before baking, in hopes of preventing them from drying out â it worked) â bake 350 degrees for 30 â 45 minutes or until cheese is melted.