In a large skillet, heat oil over medium-high heat. Sauté shallots, mushrooms, ½ teaspoon salt and ¼ teaspoon pepper for 5-7 minutes; set aside.
Spray 9×13-inch baking pan with non-stick cooking spray. In order, layer half of the croissant pieces, mushroom mixture, red peppers, cheese cubes and remaining croissant pieces.
In a large bowl, whisk together eggs, milk, mustard, herbs, ½ teaspoon salt and ½ teaspoon pepper. Pour over croissant mixture. Cover and refrigerate several hours or overnight.
Heat oven to 350 F.
Bake, uncovered, until center is set, about 55-60 minutes. Allow to set 5-10 minutes more before serving.