In a large mixing bowl, combine the baked and mashed potatoes, minced shallots, and half of the grated white cheddar. Season with salt and ground white pepper, and mix well to ensure all the ingredients are evenly incorporated.
Gradually fold in the sour cream, adding one cup at a time, until the mixture is smooth and creamy.
Preheat your oven to 350°F (175°C). Grease a large baking dish (approximately 13 x 9 inches or larger) with oil, ensuring the surface is well-coated.
Transfer the potato mixture to the greased baking dish, spreading it evenly. Sprinkle the remaining grated white cheddar on top, creating a luscious cheesy layer.
Bake the dish in the preheated oven for approximately 25 minutes, or until the potatoes are hot and the cheese is lightly browned to perfection.
Remove from the oven and allow it to cool for a few minutes before serving. The melted cheese on top will be gooey and irresistible.