Mushroom and Spinach French Toast Bake

Butter a 9"-x-13" casserole dish. Set aside.
Heat oil in a large nonstick skillet over medium heat and cook the mushrooms, stirring occasionally, until lightly golden brown, 6 to 8 minutes.

While the mushrooms are cooking, lay out half the challah slices in the buttered dish. Sprinkle with shredded cheese. Add the spinach.

In a bowl, beat eggs with milk and 1/2 teaspoon each salt and pepper. Pour egg mixture over ingredients in casserole dish. Lay out the remaining challah slices. Cover with plastic wrap, pressing down on the challah. Refrigerate overnight

Preheat oven to 375 degrees F, Remove casserole from fridge and replace plastic wrap with foil. Bake for 20 minutes, then remove foil and bake until eggs are puffed and just set, 25 to 30 minutes more.