Preheat oven to 350 degrees F (175 degrees C). Lightly grease an 8x8-inch 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 skillet over medium heat. Lightly fry the garlic, red and green pepper, carrots, and zucchini, stirring frequently, for 3 to 5 minutes.
In a mixing bowl, combine the 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.