Preheat oven to 350 degrees.
Warm the mashed potatoes. You can make them fresh or use leftover mashed potatoes. Mix with the milk, 6 tablespoons of the butter, cream cheese, green onions,& 2 cups of the cheddar cheese; season to taste with salt and pepper. Set aside.
Melt the remaining 4 tablespoons of the butter in a large skillet. Sauté the onion in the butter for about 7-10 minutes, or until they are tender and beginning to caramelize. Add in the minced garlic and cook for another minute. Remove from the heat.
Place 1/2 of the mashed potatoes into the bottom of a 9x13 inch baking dish followed by a layer of lasagna noodles. Repeat this with the remaining potatoes and another layer of noodles. Then pour 3/4 of the onion and garlic butter over the noodles, sprinkle on 3/4 of the cooked and crumbled bacon, then another layer of noodles. Pour the remaining onion garlic butter mixture over the top & then top it off with the remaining cup of cheese. Season with salt and pepper to taste.
Bake, uncovered, at 350 degrees F for 30 to 45 minutes, or until the cheese is melted and bubbly. Garnish with the rest of the cooked & crumbled bacon. Serve with sour cream and chopped fresh green onions or chives, if desired.