Follow the steps in order for the best result.
Kick off your culinary creation by cooking the 10 ounces of frozen chopped spinach according to the package directions, typically in lightly salted water.
Once cooked, it's crucial to drain the spinach exceptionally well. Press out every bit of excess moisture using a spoon or clean kitchen towel to prevent a soggy quiche.
In a medium skillet, melt the two tablespoons of unsalted butter over medium heat. Add the finely chopped yellow onion and celery, then sauté them gently for about 5 to 7 minutes until the onion becomes translucent and tender.
Carefully spread the creamy 1/2 cup of cottage cheese evenly across the bottom of your 9-inch deep-dish pie crust, creating a delightful base layer.
In a bowl, combine the thoroughly drained spinach with the sautéed onion and celery mixture. Distribute this vibrant green filling uniformly over the cottage cheese layer in the pie shell.
Generously sprinkle the 1 cup of freshly grated Swiss cheese over the spinach mixture, ensuring an even coating for that classic cheesy finish.
In a separate mixing bowl, whisk together the 6 large eggs, 1/2 cup whole milk, 1 teaspoon sea salt, 1/2 teaspoon freshly ground black pepper, and 1/4 teaspoon ground nutmeg until the mixture is smooth and fully incorporated. Gently pour this rich custard over the entire filling in the pie crust.
Bake your quiche in a preheated oven at 350°F (175°C) for approximately 40 to 45 minutes, or until the top is golden brown and a knife inserted into the center emerges clean, indicating it's perfectly set and ready to enjoy.