Prep Potatoes: Pour canned potatoes into a large colander and drain, then lightly rinse. Set aside.
Shred Cheeses: Shred the white cheddar, sharp cheddar, and colby cheddar cheeses into a large bowl and set aside.
Preheat your oven to 350°F. Lightly grease a large baking dish with butter or cooking spray.
Prepare the Cheese Sauce: In a medium saucepan, melt the stick of butter over medium low heat. Sprinkle the flour over the melted butter and whisk continuously for about 1 minute to form a roux (a thick paste). Do not burn.
Add Cream: Gradually add the heavy cream to the roux, whisking continuously to prevent lumps. Cook until the mixture starts to thicken, about 2-3 minutes.
Add Cheeses: Reduce the heat to low. Stir 3 cups of the mixed cheeses until melted and smooth. (Reserve the additional 3 cups for layering).
Add Seasonings: Season the cheese sauce with paprika, celery salt, ground mustard, salt, black pepper, garlic powder, and onion powder. Adjust the seasoning to taste with salt. Remove from heat.
Layer the Potatoes and Cheese Sauce: Arrange half of the sliced potatoes in the prepared baking dish. Pour half of the cheese sauce over the potatoes, spreading evenly. Layer the remaining potatoes on top, followed by the rest of the cheese sauce, making sure to cover all the potatoes.
Bake: Cover the baking dish with aluminum foil and bake in the preheated oven for 30 minutes. After 30 minutes, remove the foil and continue baking for an additional 30-35 minutes or until the top is golden brown and bubbly.
Rest and Serve: Let the scalloped potatoes rest for 5-10 minutes before serving. This allows the sauce to thicken slightly and makes serving easier.
*Adjust: Feel free to adjust the seasoning to your taste, adding more or less of the spices as you prefer. Canned Potatoes need to be seasoned generously but feel free to adjust the salt amount as desired!