Part 1: Preparing the Potato and Veggie Base

1. Roast the Potatoes: Preheat your oven to 400°F (200°C). Wash and dice the potatoes into uniform ½-inch cubes (peeling is optional). On a large baking sheet, toss the diced potatoes with 1 tablespoon of olive oil, ½ teaspoon of salt, and ¼ teaspoon of black pepper. Spread them into a single, even layer. Roast for 20-25 minutes, flipping halfway through, until the potatoes are tender and lightly golden brown at the edges. Pre-cooking the potatoes ensures they are perfectly cooked through in the final casserole.

2. Prepare the Baking Dish: While the potatoes are roasting, generously grease a 9×13 inch glass or ceramic baking dish. Reduce the oven temperature to 375°F (190°C) for the final bake.

3. Squeeze the Spinach (The Most Important Step!): Place the thawed spinach in the center of a clean kitchen towel, cheesecloth, or several layers of heavy-duty paper towels. Gather the ends and twist, squeezing as hard as you can to remove every last drop of water. You should be left with a dense, compact ball of spinach. This step is absolutely critical to prevent a soggy casserole.

4. Sauté the Aromatics: In a medium skillet over medium heat, add the chopped yellow onion. Cook for 5-7 minutes until softened and translucent. Add the minced garlic and cook for another minute until fragrant. Remove from heat.

5. Layer the Casserole: Spread the roasted potatoes evenly across the bottom of the prepared baking dish. Crumble the dry spinach over the potatoes, followed by the cooked onion and garlic mixture. Sprinkle 1 ½ cups of the shredded cheese evenly over the top of the vegetables.

Part 2: Assembling and Baking the Casserole

1. Make the Egg Custard: In a large bowl, crack all 12 eggs. Add the cup of milk, the remaining ½ teaspoon of salt, ¼ teaspoon of black pepper, and the optional smoked paprika. Whisk vigorously for about a minute, until the eggs are completely broken down, the mixture is uniform in color, and slightly frothy. A thorough whisking incorporates air and leads to a fluffier casserole.

2. Pour and Combine: Gently pour the whisked egg mixture evenly over the layers of potatoes, spinach, and cheese in the baking dish.

3. Top with Cheese: Sprinkle the remaining 1 cup of shredded cheese over the top of the casserole.

4. Bake to Golden Perfection: Place the baking dish in the preheated 375°F (190°C) oven. Bake for 35-45 minutes. The casserole is done when the center is set (doesn’t jiggle when gently shaken) and a knife inserted into the middle comes out clean. The top should be puffed up and beautifully golden brown.

5. Let It Rest: This step is crucial. Remove the casserole from the oven and let it rest on a wire rack for at least 10-15 minutes before slicing and serving. This allows the casserole to set up firmly, making it much easier to cut into clean, neat squares.