Heat oven to 350 degrees. Lightly coat a 3 quart shallow baking dish with non-stick spray. Cook potatoes in water to cover for 20 - 25 minutes, until tender when pierced. Drain in a colander and return to pot.
Add butter to potatoes, mash with a potato masher until smooth. Stir in 2 cups cheese, milk, sour cream and eggs until blended.
Then add 1/2 cup onions, half of the bacon, salt and pepper. Spoon into prepared baking dish. Sprinkle with remaining cheese and bacon on top.
Bake, uncovered, 40 minutes or until top is light golden. Sprinkle with remaining onions.