Tomato and Spinach Strata with Three Cheeses from Lighter, Quicker, Better: Cooking for the Way We E

Ingredients

• 1½ tablespoons chopped thyme leaves
• ¾ pound fresh or part-skim mozzarella cheese, sliced very thin
• 1 recipe Basic Chunky Tomato Sauce
• 2 loaves day-old Italian bread, with crust, preferably whole wheat, sliced ⅛ inch thick (32 to 40 thin slices)
• 6 cups skim milk, or as needed
• 1 pound part-skim ricotta cheese
• Freshly grated nutmeg
• 10 to 12 ounces mushrooms, sliced ⅛ inch thick (5 to 6 cups)
• Salt
• Freshly ground black pepper
• 2 medium-large onions, coarsely chopped (3 to 3½ cups)
• 2 tablespoons olive oil
• 3 pounds fresh spinach, stems trimmed, or 3 packages (10 ounces each) frozen spinach, thawed and squeezed dry
• ⅓ cup freshly grated parmesan cheese
• 1½ tablespoons chopped basil leaves
• 1½ tablespoons chopped parsley
• 5 large eggs
• 4 large egg whites