Preheat oven to 350°F (175°C). Lightly toast the croissant cubes on a baking sheet for 5 minutes. Set aside.

In a large bowl, whisk together eggs, milk, Dijon mustard, nutmeg, salt, black pepper, and cayenne. Set aside.

Heat 1 tablespoon oil or butter in a skillet over medium heat. Sauté shallot until soft. Add spinach, season with salt and pepper, and cook for 2-3 minutes. Remove from heat.

Grease a 9x13-inch baking dish. Layer half of the croissants, followed by half the spinach, Gruyere, and feta. Pour half of the custard over. Repeat the layers. Press down gently.

Cover with foil and refrigerate for at least 2 hours or overnight.

Let the strata sit at room temperature for 30 minutes. Bake covered for 30 minutes, then uncover and bake for 20-30 more minutes until set and golden brown.

Let rest for 5 minutes before serving.