We've substituted polenta slices with lasagna noodles in this meatless entrée. The lasagna can be assembled up to two days in advance, covered, and stored in the refrigerator. If you make it ahead, bake for an additional 10 minutes while still covered.
2 (16-ounce) tubes polenta, each cut into 20 (1/4-inch-thick) slices
How to Make It
Combine porcini and hot water in a small bowl. Let stand 30 minutes. Drain in a sieve over a bowl, reserving 1/4 cup mushroom liquid. Rinse and drain porcini; finely chop.
Preheat oven to 375°
Heat oil in a large nonstick skillet over medium-high heat. Add onion; sauté 2 minutes. Add the porcini, portobello, shiitake, and button mushrooms. Sprinkle mushrooms with salt; sauté 8 minutes.
Combine the reserved mushroom liquid, wine, tomato paste, rosemary, oregano, and tomatoes in a medium saucepan. Bring to a boil; reduce heat, and simmer 5 minutes.
Combine 1 cup mozzarella, thyme, ricotta cheese, and egg whites in a bowl; set aside.
Arrange 15 polenta slices in the bottom of a 13 x 9-inch baking dish coated with cooking spray. Top with 2 1/2 cups mushroom mixture, 1 cup tomato sauce, and 1 1/4 cups ricotta mixture. Repeat layer with 15 polenta slices, remaining mushroom mixture, 1 cup tomato sauce, and the remaining ricotta mixture. Top with the remaining polenta slices and tomato sauce. Sprinkle the lasagna with 1 cup mozzarella. Cover and bake at 375° for 45 minutes.
Uncover and bake an additional 15 minutes. Let stand 5 minutes.