Something magical happens when you cook butter just past its melting point. Trust us, the brown, nutty transformation is no illusion, and when mixed with panko breadcrumbs, it elevates the topping for this mac and cheese to stratospheric heights. Not that the creamy triumvirate of Parmesan, cheddar, and mozzarella can't hold its own. Serve this mac next to the delicious mélange of Parmesan-roasted zucchini and Roma tomatoes, and watch this meal magically disappear.
Add pasta to boiling water and cook until al dente, 7-9 minutes. Reserve ¾ 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.
Roast the Vegetables
Trim zucchini ends, halve lengthwise, and cut into ½" slices on an angle. Core Roma tomatoes and cut into ½" dice. Place zucchini and tomatoes on prepared baking sheet and toss to coat with 1 tsp. olive oil, ¼ 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.
Make the 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 ¾ tsp. salt until smooth and immediately remove from burner. Stir in pasta.
Make the Brown Butter Panko
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.
Bake the Mac and Cheese
Transfer mac and cheese to prepared casserole dish. Sprinkle panko-brown butter mixture on top and bake until golden brown, 7-10 minutes.
Plate the Dish
Reheat vegetables in oven 2-3 minutes if necessary. Serve vegetables alongside mac and cheese, family style.