Preheat your oven to 350°F (175°C). Lightly toast the croissant cubes on a baking sheet for about 5 minutes until golden.

Whisk together the eggs, milk, Dijon mustard, salt, black pepper, nutmeg, and cayenne in a large mixing bowl until smooth and slightly frothy.

Heat olive oil or butter in a skillet over medium heat. Sauté shallots for 3-4 minutes until translucent. Stir in spinach and cook for 3-5 minutes.

Grease a 9x13 baking dish and layer half of the toasted croissant cubes, followed by half of the spinach mixture, half of the gruyere cheese, and half of the egg custard. Repeat the layering.

Gently press down on the layers with a spatula to ensure the croissants soak up the egg mixture effectively.

Cover the dish with foil and refrigerate for at least 2 hours, or preferably overnight.

Let the strata sit at room temperature for about 30 minutes before baking.

Bake covered for 30 minutes, then remove the foil and bake for an additional 20-30 minutes until set and golden.

Let the Croissant Strata sit for 5 minutes before cutting into squares and serving.