Preheat oven to 350°.
Spoon 1/2 cup marinara sauce into an 8-inch square baking dish to cover bottom, and set aside.
Heat oil in a large nonstick skillet over medium-high heat. Add onion and bell pepper; sauté 4 minutes or until tender. Stir in sausage; cook 2 minutes. Add mushrooms, zucchini, and garlic; sauté 2 minutes or until mushrooms are tender, stirring frequently. Add remaining marinara sauce; reduce heat, and simmer 10 minutes.
Arrange 9 polenta slices over marinara in baking dish, and top evenly with half of vegetable mixture. Sprinkle 1/4 cup of cheese over vegetable mixture; arrange remaining polenta over cheese. Top polenta with the remaining vegetable mixture, and sprinkle with remaining 1/4 cup cheese.
Cover and bake at 350° for 30 minutes. Uncover and bake an additional 15 minutes or until bubbly. Let stand 5 minutes before serving.
"Sun-dried tomato and garlic polenta is also good in this because the flavors complement the vegetables. The lasagna is delicious with or without the sausage, or you could substitute lean ground beef." -Angela McKinlay, Everett, WA