Cheesy Egg Potato Bake | Mama's Cooking

Ingredients

• 3 tablespoons unsalted butter, softened, plus more, to taste, for brushing
• 1 onion, chopped
• salt, to taste
• pepper, to taste
• 1 pound curly-leaf spinach, thick stems removed, sliced
• 2 teaspoons fresh thyme leaves
• 1 pound Yukon Gold potatoes, peeled and diced 1/2-inch-thick
• 1 1/2 cups whole milk
• 12 large eggs
• 1/4 teaspoon nutmeg, freshly grated
• 2 cups fontina, coarsely grated, divided