Cheesy Polenta Casserole

2 teaspoons olive oil
2 green bell peppers, chopped
2 cloves minced garlic
4 green onions
1 large jar marinara sauce
1 cup shredded Fontina cheese
1 cup shredded mozzarella cheese
2 (16 oz.) tubes read made polenta, cut into ¼ inch slices
2/3 cup grated Parmesan cheese
1/2 cup walnuts
Preheat oven to 350 degrees. Spray a casserole dish with non stick cooking spray. Heat oil in a saucepan over medium heat. Stir in green pepper, garlic, and green onions; cook until pepper softens. Pour in marinara sauce and simmer for 5 to 7 minutes. Mix together Fontina and mozzarella cheese. Spread a thin layer of sauce over baking dish. Arrange 1/3 of polenta slices in the dish and top with 1/3 cheese mixture. Spread a thin layer of sauce over cheese. Repeat layers twice. Top casserole with Parmesan cheese. Bake for about 25 minutes until cheese is bubbly and golden brown. Top with walnuts.