Cook pasta according to package directions. Meanwhile, heat oil in a large frying pan over medium high heat. Add sausages and cook 7-10 minutes, or until cooked through. Remove from pan and set aside. Drain all but 1 tablespoon of fat from pan.
Preheat oven to 375F. Reduce heat to medium and add onion and garlic. Cook, stirring frequently, for 3 minutes or until onion is tender. Add zucchini, peppers, mushrooms, Italian seasoning, salt and pepper to taste. Cook, stirring occasionally, just until peppers are barely tender, about 5 - 7 minutes. Stir cheeses together.
Drain pasta when cooked and turn into a casserole dish along with sausages, vegetables plum tomatoes and half of the cheese mixture. Toss together. Sprinkle with remaining cheese. Bake covered, in a preheated oven for 20 minutes. Uncover and continue to bake for 10 - 15 minutes until casserole is hot.