Heat the oven to 375°F and grease a 9-by-13-inch baking dish. Bring a large pot of generously salted water to a boil.

Cook the pasta until just shy of al dente, usually 1–2 minutes less than the package directions. Drain it well, but do not rinse, so the sauce can cling to the pasta.

While the pasta cooks, heat the olive oil in a large skillet. Add the broccoli, cauliflower, zucchini, red bell pepper, and garlic, then cook for about 5–7 minutes until the vegetables are slightly tender but still crisp.

In a bowl, stir together the Alfredo sauce, milk, Italian seasoning, salt, and black pepper until smooth and evenly combined.

Combine the drained pasta, sautéed vegetables, Alfredo mixture, 1/2 cup of the mozzarella, and Parmesan. Stir gently until the sauce coats the pasta and the vegetables are distributed throughout.

Transfer the mixture to the greased baking dish and spread it into an even layer. Sprinkle the remaining 1/2 cup mozzarella over the top.

Cover the dish with foil and bake for 15 minutes. Remove the foil and continue baking for another 8–12 minutes, until the casserole is hot and bubbling and the cheese is lightly browned.

Let the pasta bake rest for 5 minutes before serving. This gives the sauce a moment to settle while keeping each scoop creamy and easy to serve.