Add pasta to boiling water and cook until al dente, 7-9 minutes. Reserve 3/4 cup pasta water. Drain pasta in a colander and rinse briefly under cold water. Reserve pot; no need to wipe clean. While pasta cooks, roast vegetables.

Trim zucchini ends, halve lengthwise, and cut into 1/2" slices on an angle. Core Roma tomatoes and cut into 1/2" dice. Place zucchini and tomatoes on prepared baking sheet and toss to coat with 1 tsp. olive oil, 1/4 tsp. salt, and a pinch of pepper. Spread into a single layer and sprinkle with half the Parmesan (reserve remaining for sauce). Roast until golden brown and tender, 15-18 minutes. While vegetables roast, make mac and cheese.

Add milk, flour, and reserved pasta water to pot used to cook pasta. Bring to a boil over medium-high heat, stirring often. Reduce to medium heat and simmer until thick enough to coat the back of a spoon and draw a line through it, 3-5 minutes. Whisk in cheddar, mozzarella, remaining Parmesan, and 3/4 tsp. salt until smooth and immediately remove from burner. Stir in pasta.

Add butter to a small pan over medium heat and cook, stirring occasionally, until it begins to smell nutty, turn golden, and brown flecks appear, 5-6 minutes. Stir in panko breadcrumbs to coat and immediately transfer to a small mixing bowl.

Transfer mac and cheese to prepared casserole dish. Sprinkle panko-brown butter mixture on top and bake until golden brown, 7-10 minutes.

Reheat vegetables in oven 2-3 minutes if necessary. Serve vegetables alongside mac and cheese, family style.