Preheat oven to 375°F and grease a 9x13-inch baking dish. I use the same dish every week.
Sauté onion in 1 tbsp butter until soft, about 4 minutes. Let cool slightly.
In a large bowl, beat eggs with milk, ricotta, garlic powder, salt, and pepper.
Stir in thawed hashbrowns, spinach, sautéed onion, and 1/2 cup mozzarella.
Spread mixture into the dish, dot with remaining butter, and sprinkle with Parmesan and remaining mozzarella.
Bake 35–40 minutes until set and edges are golden. Let rest 5 minutes before cutting.