Prepare for baking by preheating your oven to 375°F or 190°C (375°F). Lightly grease a 3-quart or 2. 8-liter rectangular baking dish.
In a large Dutch oven or pot, cook the macaroni in boiling, salted water for 5 minutes.
Add the frozen vegetables, sliced zucchini, and chopped red pepper to the pot and continue cooking for the final 3 minutes, or until the pasta is al dente.
Drain everything well and return the pasta and veggie mixture to the hot pot.
While the pasta cooks, create the creamy sauce.
In a medium saucepan, whisk together the evaporated milk, vegetable broth, flour, oregano, salt, garlic powder, and black pepper until smooth.
Cook over medium heat, stirring constantly, until the sauce thickens and begins to bubble.
Pour this luscious sauce over the drained pasta and vegetables in the Dutch oven.
Add 1/2 cup or 50g (1.76 oz) of the Parmesan cheese and stir until everything is evenly coated and gloriously cheesy.
Transfer the macaroni mixture into your prepared baking dish, spreading it into an even layer.
Bake uncovered for 25 minutes. Carefully remove the dish from the oven, arrange the fresh tomato slices on top, and sprinkle with the remaining 1/4 cup or 25g (0.88 oz) of cheese. Return to the oven and bake for another 5 minutes, or until the cheese is melted and the casserole is bubbly.
Let it stand for 5 minutes to set slightly before serving.