In a large skillet over medium heat, saute ham, onions, garlic, peppers, and mushrooms until vegetables are crisp-tender. Add spinach and cook a few minutes more, until just wilted. Season with salt and pepper, to taste.

Generously grease a 9x13 inch baking pan. Add the croutons to the bottom of the pan in an even layer.

In a large bowl, whisk the eggs, Herbs de Provence, milk, and 1/4 cup shredded cheese together. Pour half of this egg mixture over the croutons. Top with the ham and vegetable mixture, then the remaining egg mixture. Sprinkle the remaining cheese over top.

Cover casserole with plastic wrap, followed by a layer of aluminum foil. Freeze until ready to use (up to 3 months). To bake: thaw the casserole in the refrigerator overnight. Allow casserole to stand at room temperature for about 15 minutes before baking.

Preheat oven to 375 degrees. Bake the casserole, uncovered, until the top is golden and a toothpick inserted in the center comes out clean-- about 40-45 minutes.  Let stand for 10 minutes before serving.