Give the bottled zesty Italian dressing a good shake, then slowly pour it evenly over the macaroni and cheese mixture, making sure to cover as much of the pasta as possible. Gently tilt the pan back and forth so the dressing seeps down around the noodles.
Cover the pan tightly with aluminum foil to trap in steam. This is important because the pasta will cook in the liquid from the dressing.
Bake on the center rack for 35 to 40 minutes. Carefully remove the foil and check a piece of macaroni from the center of the pan; it should be just tender. If the pasta still seems a bit firm and looks dry on top, pour up to 1/2 cup water evenly over the pasta, re-cover with foil, and bake for another 10 minutes.
Once the pasta is tender and most of the liquid is absorbed into the noodles and cheese, remove the foil and sprinkle the remaining 1/2 cup shredded mozzarella evenly over the top.
Return the uncovered pan to the oven and bake for 8 to 10 more minutes, or until the cheese on top is melted and lightly golden around the edges.
Let the macaroni rest for about 5 to 10 minutes before serving. This helps it set up a bit and makes it easier to scoop into bowls.
Taste and adjust at the table with a little extra Parmesan, salt, or black pepper if needed, then serve warm.
For extra protein, you can stir in about 1 to 1 1/2 cups of cooked, diced chicken or ham right after you add the cheeses, before pouring on the dressing. If you like a creamier texture, replace 1/2 cup of the zesty Italian dressing with 1/2 cup of half-and-half or heavy cream; just be aware the flavor will be a little less tangy.
To sneak in some veggies, scatter 1 to 1 1/2 cups of frozen peas, corn, or mixed vegetables over the macaroni before adding the cheeses and dressing—no need to thaw first. For a spicier version, use a spicy Italian dressing and add crushed red pepper flakes or a pinch of cayenne. If you prefer a crispier top, broil the finished dish for 1 to 2 minutes, watching closely so the cheese doesn’t burn.
Food safety tips: Always check that any add-ins like meat are fully cooked before mixing into the casserole; this recipe doesn’t bake long enough to safely cook raw meat. Keep leftover macaroni refrigerated in a shallow container and eat within 3 to 4 days, reheating until steaming hot before serving. Because this dish uses a vinegar- and oil-based dressing, don’t leave it out at room temperature for more than 2 hours (or 1 hour if your kitchen is very warm) to avoid bacterial growth.
