Bring a stockpot of water to a boil. Add asparagus and cook uncovered for 2 minutes. Drain; then plunge asparagus into ice water. When cool, pat dry and cut into 1-inch pieces.
Coat a shallow 2-quart baking dish with cooking spray (a 13-inch-by-9-inch pan works well). Cut about 4 croissants in half lengthwise and place in the bottom of the dish cut side up. Gently flatten and cut croissants as necessary to fit in a continuous layer.
Scatter 1 cup of the Fontina, asparagus, bell peppers, and scallions over the croissants. Cut remaining croissants into 1-inch cubes and scatter over the top.
In a large bowl, whisk together the eggs, milk, parsley, salt, pepper, mustard, and nutmeg. Pour evenly over the croissants, pressing gently, so the bread absorbs the liquid. Scatter the remaining 1 cup of cheese over the top. Cover with plastic wrap and refrigerate for at least 4 hours or overnight.
Remove the dish from the refrigerator 20 minutes before baking. Preheat the oven to 350ºF. Uncover the dish, and bake until eggs are set in the center and the croissants are lightly browned, about 45 minutes.
Tip: If you prefer, replace the asparagus with cooked spinach and mushrooms - or any vegetable you like. You can also replace the Fontina with sharp provolone, mozzarella, Gruyère, or sharp cheddar.