Step 1

Preheat oven to 375°F. Coat a 9x13-inch casserole dish with cooking spray. Set aside.

Step 2

Heat olive oil in a skillet on medium-high heat. Sauté onions until translucent. Transfer onions to a large bowl.

Step 3

Add spinach to bowl with onions. Add sour cream, mayonnaise, beaten eggs, garlic powder, salt, pepper, cayenne, Parmesan and Romano cheese. Stir until spinach mixture is well combined.

Step 4

Layer slices of potato in an overlapping pattern on the bottom of the prepared casserole dish.

Step 5

Pour spinach mixture over top of potatoes. Top with 1 cup mozzarella cheese. Cover with foil and bake for 20 minutes. Remove foil and bake for an additional 15-20 minutes or until cheese on top is golden brown on the edges.

Step 6

Rest for 5 minutes before serving.