Preheat the oven to 350℉. Grease a deep 9x13-inch casserole with cooking spray.

Melt the butter in a skillet over medium heat. Add the onions and saute until translucent and starting to brown. Remove from the heat and set aside.

In a medium mixing bowl, mix together the cheddar and mozzarella cheese.

Place three lasagna noodles lengthwise on the bottom of the casserole dish.

Spread one-third of the mashed potatoes on the noodles.

Spread one-third of the onions over the potatoes.

Sprinkle one-quarter of the cheese over the onions.

Repeat steps 4 to 7 twice more.

Add a final layer of noodles and top with bacon and remaining cheddar cheese. Sprinkle salt and pepper on top.

Cover the baking dish with aluminum foil. Bake for 30 minutes.

Remove the foil and return to the oven for 10 more minutes to melt the cheese.

Let rest for 10 minutes before serving. Add garnishes if desired like green onions, chives, sour cream, etc.