Trim ends of zucchini and squash and cut diagonally into ½” slices.
Peel onion and cut into ½” slices.
Cut tomatoes into ½” slices.
Arrange vegetables on a sheet pan. Drizzle with olive oil. Sprinkle Parmesan cheese, minced garlic, Italian seasoning, salt and pepper over tops of vegetables.
Bake at 400°F for 30-35 minutes.
{Optional} Place pan under broiler for 3-4 minutes or until tops of vegetables turn crispy. (Note: Watch pan closely in this step to prevent vegetables from burning!)