Preheat your oven to 375℉ and grease a 2-quart baking dish.
In a small skillet, melt the butter over medium-low heat. Sauté the diced shallot until it becomes soft and translucent, about 4 minutes. Add the minced garlic and cook for an additional 30 seconds. Remove from heat.
Using a box grater or a food processor, grate the cooled baked potatoes with the skins on.
In a large mixing bowl, combine the sour cream, salt, black pepper, 2 cups of the shredded cheddar cheese, and the sautéed shallots and garlic. Gently fold in the grated potatoes, ensuring the mixture is well combined while maintaining the shredded potato texture.
Transfer the potato mixture into the prepared baking dish, piling it lightly. Evenly sprinkle the remaining cheddar cheese and Parmesan over the top.
Bake in the preheated oven for 25-30 minutes or until the top is golden brown.