Prepare the Spinach: In a skillet over medium heat, drizzle a tablespoon of olive oil, then add fresh spinach. Sauté for about 3-5 minutes until wilted and vibrant green, stirring occasionally. Once softened, remove from heat and set aside.

Make the Custard: In a large mixing bowl, whisk together 6 eggs, 1 cup of milk, and a pinch of cayenne pepper along with salt and pepper to taste. Mix until fully combined and frothy, about 2 minutes.

Layer the Ingredients: Preheat your oven to 350°F (175°C). In a greased 9x13-inch baking dish, layer 4 cups of torn croissant pieces, followed by sprinkling with sautéed spinach, crumbled feta, and grated gruyere cheese.

Combine and Soak: Carefully pour the egg custard over the layered ingredients, ensuring it saturates the croissants and coats the spinach and cheese. Press down gently, then cover with plastic wrap and refrigerate for at least 1 hour or overnight.

Bake the Strata: Remove from the refrigerator, uncover, and bake in the preheated oven for 50 minutes, until light golden brown and set.

Cool and Serve: Let the strata cool for about 10 minutes before slicing and serving warm.