Bring a large saucepan of lightly salted water to a boil. Add pasta and cook until almost tender; about 10 minutes.
Fry the sausages in a large frypan over medium heat turning frequently until cooked through; about 15 minutes. Remove from the frypan, cool slightly and slice into rounds.
Add the garlic, fennel and onion to the frypan and season with salt and pepper. Cook and stir for about 5 minutes then add the roasted red capsicums, basil, sliced sausage and pasta sauce. Heat through over low heat until warmed.
Combine the pasta with the sauce and vegetables in a 20x30cm baking dish. Spread the mozzarella, Parmesan and Asiago cheeses over the top. Garnish with a few fennel leaves left from the bulb. Cover with aluminium foil.
Bake for 30 minutes in the preheated oven then remove the aluminium foil. Set the oven to grill and cook for another 5 minutes or until cheese is browned.