In a medium frying pan, melt 1 tablespoon of unsalted butter over medium-low heat. Add the sliced cremini mushrooms and sauté for about 10-12 minutes until they're tender and golden brown. Once done, remove from the pan and sauté the spinach for 2-3 minutes until wilted.

Preheat your oven to 350°F (180°C). Grease a 10.5x7 inch baking dish with unsalted butter. Slice the croissants in half lengthwise and arrange them vertically in the dish, stuffing each with the sautéed spinach and mushrooms.

In a separate bowl, whisk together 6 large eggs and 1 cup of whole milk. Stir in half of the grated Gruyère cheese, and season with salt and pepper.

Pour the egg mixture over the croissant layers, allowing it to seep through. Top with the remaining grated Gruyère cheese.

Cover the dish with aluminum foil and bake for 20 minutes. Remove the foil and continue baking for an additional 10-15 minutes until the eggs are set and the cheese is golden.

Allow the casserole to cool for a few minutes before slicing and serving warm.