This sausage-and-pepper-filled recipe can easily be doubled to serve both dinner one night and lunch the next day.
Saute sweet and hot sausage in a large skillet over medium heat; stir with a wooden spoon to break up the pieces until the sausage is well browned, about 8 minutes. Set sausage aside. Add the red and yellow pepper to the sausage fat, and saute until softened, 6 to 7 minutes. Set aside. Add the onions to the remaining fat, adding olive oil if necessary. Reduce heat to medium low, and saute until onions are very soft but not brown, 10 to 15 minutes. Set aside.
Heat the olive oil in a medium pot over medium heat. Add the garlic and crushed red-pepper flakes, and saute until garlic is translucent. Add the canned tomatoes and tomato paste. Reduce heat to low, and let the mixture simmer until it has thickened and reduced, about 30 minutes. Add the reserved cooked sausage. Season with salt and pepper. Remove from heat, and set aside.
Heat oven to 375 degrees. Spoon 1/2 cup tomato sauce into the bottom of an 8-by-8-inch oven-safe glass dish. Place a layer of fresh lasagna sheets on top. Spread half the ricotta over pasta; cover with spinach. Cover with second layer of fresh pasta. Sprinkle pasta with half the fresh mozzarella and half the Parmesan. Cover with 1 cup sauce and a third layer of pasta. Spread pasta with remaining ricotta, cover with sauteed onions and peppers. Cover with a final layer of pasta, top with remaining sauce, sprinkle with remaining mozzarella and Parmesan. Place in the oven to bake until sauce is bubbling and top is golden brown, 35 to 45 minutes. Serve.