Step 1

Preheat the oven to 350 degrees F and generously butter a 9x13 pan.

Step 2

Bring a large pot of water to a rapid boil. Once boiling, salt with 1 tablespoon of kosher salt and cook the lasagna noodles according to package directions. Drain and set aside to assemble the lasagna.

Step 3

Place the leftover mashed potatoes in a heat-safe bowl and cover with a damp paper towel. Microwave for 3-4 minutes to loosen up the mashed potatoes. If needed, add 1/4 to 1/2 cup of warm broth or milk to make the potatoes creamier.

Step 4

Spread 2 cups of the heated mashed potatoes on the bottom of the greased casserole dish. Sprinkle with 1 cup of cheese. Place 3 noodles over the layer of cheese. Repeat layers. For the final layer, spread the remaining potatoes over the last layer of noodles. Sprinkle with the remaining cheese and bacon, if using. Cover the casserole dish with foil.

Step 5

Bake for 40 minutes and then remove the foil. Bake for another 20 minutes, or until the cheese is browned.

Step 6

Remove from the oven and allow to cool for 10-15 minutes before slicing. Serve as is, or with caramelized onions and a dollop of Greek yogurt or sour cream.