Preheat the oven to 350°F. Lightly oil an 8'' x 11'' baking dish.
Take the roll of pre-cooked polenta and slice it into 12 equal rounds. Lay the polenta rounds on the bottom of the baking dish, completely covering it.
Then take ⅓ of a medium sized onion and finely chop it. Set aside. Roughly chop 2 cups of spinach.
In a mixing bowl, crack 8 eggs. Use a wire whisk or fork to beat the eggs until all of the yolks are broken and the mixture becomes nice and fluffy.
Then add in the garlic powder, salt, and black pepper to the eggs. Mix well.
Stir in the diced tomatoes, chopped onion, parmesan cheese, and spinach until well combined.
Pour the egg mixture over the polenta and place it on the middle oven rack.
Bake it for 45-48 minutes or until it gets a little browned on top and the center does not jiggle when moving it. (a very slight jiggle is fine, it will continue to firm up as it cools)
Serve warm.