Preheat oven to 180 degrees C. Lightly grease a square baking dish.
Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain. Meanwhile, heat oil in a large frypan over medium heat. Lightly fry the garlic, red and green capsicum, carrots and zucchini; about 3 to 5 minutes.
In a large mixing bowl, combine the mushroom soup and tuna. Stir in the vegetables and the cooked pasta. Mix to combine well. Pour into the prepared baking dish. Sprinkle the cheese and fried onions over the top.
Bake in a preheated oven for 30 minutes or until bubbly and slightly brown.