1In large heavy skillet, crumble and brown sausage until well-browned; drain off fat, set sausage aside. Wipe skillet clean; heat olive oil over medium-high heat; add sliced peppers and the sliced onion and 2 cloves of garlic. Cover and cook over medium heat, stirring occasionally, until vegetables are wilted, about 10 minutes. Add tomatoes and fennel seed. Reduce heat to low and simmer about 20-30 minutes, to develop flavors. Stir in reserved sausage. Season sauce to taste with salt and pepper; set aside.
2Squeeze all liquid from thawed spinach, set aside. In small saucepan, saute the chopped onion and 2 remaining garlic cloves in the butter until wilted, about 10 minutes. Stir in spinach and 1/4 cup of the Parmesan cheese, set aside.
3In a medium bowl, stir together the ricotta, mozzarella and 1/2 cup of the Parmesan cheese. Season with salt and pepper, to taste. Boil noodles according to package directions; drain.
4Preheat oven to 350 degrees F. Pour one-third of tomato sauce into a greased 9x13-inch baking dish; spread evenly. Arrange 3 noodles in the dish and spread one-half of the spinach over the noodles. Spread one-half of the cheese mixture evenly over the spinach. Repeat this layering process once more. Top with the remaining 3 noodles and tomato sauce. Sprinkle top of dish with remaining 1/4 cup of Parmesan cheese.
5Bake lasagna for about 1 hour, until bubbly and browned on top. Let cool for 10-15 minutes before serving.