Prepare a deep 9-by-13-inch baking pan by spraying it with cooking spray. Set it aside.
Boil enough lasagna noodles to be able to cover three layers in your pan. (My local store had long, thin lasagna noodles, so I only needed about nine of them to do the trick.)
Heat a skillet over medium heat and saute the sausage and onions, breaking apart the sausage as it cooks. Season with salt and pepper, to taste.
While the sausage cooks, add your cottage cheese, egg and 1/2 teaspoon onion powder to a bowl and mix thoroughly.
In another bowl, add the mashed potatoes, garlic, 1/2 teaspoon onion powder, garlic, salt and pepper, along with 1 cup shredded cheese and mix well to combine.
Once your sausage is cooked through, drain off any excess fat.
To assemble the casserole, start by spooning just a little (maybe a scant 1/4 cup) of your sausage/onion mixture into the bottom of your dish. Place a single layer of noodles on top. Spread about half of your cottage cheese mixture over the noodles, about half of the sausage, and then top with about one third of your mashed potatoes.
Add another layer of noodles, spread out the rest of the cottage cheese, the rest of sausage, and another third of mashed potatoes.
Finish with a final layer of noodles and the rest of the mashed potatoes on top.
Sprinkle the rest of the shredded cheddar on the top, cover the dish with foil, and bake for 30 minutes. Remove the foil and bake for another five minutes or until the cheese is melty and slightly browned.
Let the casserole cool for about 10 minutes before slicing and serving.