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Classic spinach pie made with caramelized onions, Gruyere, and Parmesan cheeses wrapped in flaky puff pastry. This easy spinach casserole features a rich, creamy filling perfect for family dinners, holiday meals, or vegetarian-friendly gatherings. Simple ingredients combine for a savory pie that’s crispy on the outside and flavorful inside.

Ingredients

4 tablespoons salted butter (1/2 stick)
2 large yellow onions, cut into ½-inch pieces
3 cloves garlic, minced
2 (10 oz.) packages frozen spinach, thawed and drained
5 large eggs, separated
1 tablespoon breadcrumbs (Italian or plain)
1 cup Gruyere cheese, shredded
¾ cup Parmesan cheese, shredded
2 sprigs fresh thyme or 2/3 teaspoon dried thyme
½ teaspoon dried sage
Salt and pepper, to taste
2 sheets puff pastry, thawed in the fridge overnight

Instructions

Prepare the Puff Pastry:

Allow the puff pastry to thaw in the fridge overnight. Do not unroll it while frozen or let it sit at room temperature for too long to maintain its ability to rise.

Caramelize the Onions:

Heat the butter in a medium saucepan over medium heat.
Cut the onions into ½-inch pieces. Avoid dicing them too finely as they will shrink during cooking.
Cook the onions for 15 minutes. Lower the heat to medium-low and continue cooking until they are brown and caramelized, about 35 minutes in total. If short on time, use one onion and cook for about 7 minutes until softened.
Add the minced garlic and cook for an additional minute.

Prepare the Filling:
5. Preheat the oven to 400°F (200°C).
6. Drain any excess water from the spinach thoroughly.
7. Prepare an egg wash: Crack one egg into a small bowl, add 1 tablespoon of water, whisk, and set aside.
8. In a large mixing bowl, whisk the remaining four eggs vigorously until well-combined to create air pockets for a fluffier consistency.
9. Add the drained spinach, breadcrumbs, Gruyere, Parmesan, thyme, sage, salt, pepper, and the caramelized onions/garlic. Mix until fully combined.

Assemble the Pie:
10. Lightly grease a 9×13-inch casserole dish. Roll out one sheet of puff pastry to fit the bottom and sides of the dish.
11. Spread the spinach filling evenly over the pastry base.
12. Roll out the second puff pastry sheet to cover the top of the pie.
13. Seal the edges using a fork to create indentations around the dish.
14. Brush the top with the egg wash, ensuring it does not drip down the sides to maintain proper rising.
15. Cut small slits on the top to allow steam to escape during baking.

Bake:
16. Bake in the preheated oven for 35 minutes, or until the top is golden brown.
17. Let the pie rest for 5 minutes before serving.

Why You’ll Love This Recipe

Rich and Comforting: Perfectly caramelized onions and a cheesy spinach filling wrapped in flaky puff pastry.
Make-Ahead Friendly: Prepare components in advance for quick assembly.
Family Approved: A hearty dish that pleases kids and adults alike.
Flexible Ingredients: Easily adaptable to suit dietary needs or ingredient availability.

Tips

Thawing Spinach: Squeeze the spinach thoroughly to remove excess water for a firmer filling.
Puff Pastry Handling: Keep pastry cool while working to prevent it from becoming too soft.
Caramelizing Onions: Patience is key; the deep flavor is worth the time.
Even Coverage: Roll the pastry slightly larger than the dish for better sealing and coverage.

Variations and Substitutions

Cheese Options: Substitute Gruyere with mozzarella or cheddar for a different flavor.
Add Protein: Include cooked, crumbled sausage or diced ham for a heartier pie.
Vegetarian: Skip the Parmesan or use a vegetarian alternative.
Herbs: Swap thyme and sage for fresh dill or parsley for a fresh twist.

FAQs

Q: Can I use fresh spinach instead of frozen?
A: Yes, sauté fresh spinach until wilted and drain excess liquid before using.

Q: How do I prevent the bottom pastry from becoming soggy?
A: Ensure the filling ingredients are well-drained and cool before assembly.

Q: Can I make this ahead of time?
A: Yes, assemble the pie up to 24 hours in advance and store it in the fridge. Bake just before serving.

Serving and Suggestions

Main Dish: Serve with a simple side salad or roasted vegetables.
Holiday Favorite: A great addition to Easter, Thanksgiving, or Christmas menus.
Leftovers: Store in the fridge for up to 3 days. Reheat in the oven to retain crispiness.