To prepare polenta, cut bell peppers in half lengthwise; discard seeds and membranes. Place pepper halves, skin sides up, on a foil-lined baking sheet; flatten with hand. Broil 15 minutes or until blackened. Place in a large zip-top plastic bag; seal. Let stand 15 minutes. Peel and cut into 1/4-inch pieces.
Combine 8 cups water and 1 teaspoon salt in a large saucepan; bring to a boil. Add polenta, stirring with a whisk until polenta begins to absorb the water (about 2 minutes). Reduce heat to low; cook 25 minutes, stirring frequently. Add bell pepper pieces and 3/4 cup Parmesan; stir until well blended. Spoon polenta mixture onto an 18 x 12-inch jelly roll pan coated with cooking spray; spread into an even layer using a spatula coated with cooking spray. Cool to room temperature on a wire rack. Cover and refrigerate overnight.
Place a large cutting board on top of polenta. Turn over and remove the polenta from pan; cut polenta in half crosswise. Set aside.
To prepare the filling, heat a large nonstick skillet over medium-high heat. Add 1/4 cup water and garlic, and cook 1 minute. Add half of spinach; cover and cook for 1 minute or until the spinach begins to wilt. Stir in remaining spinach, 1/4 teaspoon salt, and black pepper. Cover and cook for 3 minutes. Place the spinach mixture in a colander, and drain well. Combine spinach mixture, ricotta, nutmeg, and egg whites.
Preheat oven to 375°.
Spread 1/2 cup marinara into bottom of a 13 x 9-inch baking dish coated with cooking spray. Place 1 polenta sheet half over the marinara. Spread 1/2 cup marinara over the polenta. Arrange sliced mozzarella and spinach mixture over marinara, and top with 1/2 cup marinara. Place the remaining polenta sheet half over the sauce. Top with the remaining marinara; sprinkle with 3/4 cup Parmesan. Cover and bake at 375° for 35 minutes. Uncover and bake an additional 20 minutes. Let lasagna stand 15 minutes.