Spinach and Mushroom Strata

This meat-free cheese casserole, golden and bubbly, is best served straight from the oven.
Preheat the oven to 375°F. Grease a 2 1/2-liter baking dish with cooking spray. In a large skillet, melt the butter over medium-high heat. Add the onion and mushrooms; cook, stirring, until soft and golden, about 6 minutes. Remove from heat. Add the spinach, mix well, and season with salt and pepper.Place approximately 1/3 of the bread cubes on the bottom of the baking dish, then 1/2 of the vegetable mixture and 1/3 of the cheese. Repeat with 1/3 of the bread, remaining vegetables, and another 1/3 of the cheese. Top with the remaining bread and cheese.In a large bowl, whisk together the milk, eggs, and mustard. Season with salt and pepper. Pour in layers. Press down the bread mixture with the back of a spoon to moisten it.Bake until the puff pastry is set and the center is puffy and golden, 45 to 50 minutes. Serve immediately.