Butter (for the dish)
1 loaf (about 19 ounces) challah or other rich, white toasting bread, cut into thick slices
1 tablespoon butter
2 packages (10 ounces each) white or baby bella mushrooms, halved and thickly sliced
Salt and pepper, to taste
10 ounces fresh baby spinach, rinsed
2 1/2 cups whole milk
3 cups grated cheddar
1 bunch scallions, thinly sliced
1. Butter a 9-by-13-inch baking dish.
2. Line the bottom with a layer of bread; do not overlap the slices.
3. In a large skillet, melt the butter. Add the mushrooms, salt, and pepper. Cook, stirring occasionally, for 5 minutes or until the mushrooms release their juices. Turn up the heat and continue cooking, stirring often, until the liquid evaporates. Set aside to cool.
4. In a large microwave-safe bowl, microwave the spinach, covered with a paper towel, for 2 minutes, turning once halfway through cooking. Set aside to cool. Chop coarsely.
5. In a bowl, whisk the eggs and milk with a generous pinch each of salt and pepper.
6. Sprinkle half the mushrooms over the bread in the dish. Add half the spinach and sprinkle with 1 1/2 cups of cheese. Ladle half the egg mixture into the dish.
7. Add another layer of bread, fitting the pieces neatly so they make a layer with no overlapping pieces. Add mushrooms and spinach. Sprinkle with scallions. Ladle the remaining egg mixture on top. With your hands, press the bread into the dish so it is completely covered with egg.
8. Sprinkle with the remaining 1 1/2 cups cheese. Cover with plastic wrap and refrigerate for at least 3 hours or as long as 1 day.
9. Remove from the refrigerator 1 hour before baking.
10. Set the oven at 400 degrees. Remove the plastic wrap and set the dish on a rimmed baking sheet. Bake for 45 minutes or until puffed and brown. Cut into squares for serving.