1. Boil potato chunks in salted water until tender; about 12-15 minutes, depending on the size of your chunks. Drain. Place in a large mixing bowl and mash with a potato masher.
2. Sauté onion in 1 tablespoons butter until transparent.
3. Preheat oven to 375°.
4. Combine mashed potatoes with the 1/3 cup butter, milk, sautéed onion, salt and pepper. Mash to combine together.
5. Add cheese and stir.
6. Fold eggs into potato mixture with spatula. Spray a 2 1/2 quart casserole dish with non-stick spray and pour potato mixture into it.
7. Bake at 375° for 45 minutes or until puffy and brown.