Preheat the oven to 400 degrees Fahrenheit, and spray a 9 x 13-inch baking dish with nonstick cooking spray.
Add 1 pound lean ground beef to a large skillet and cook over medium heat until fully browned, about 5 to 7 minutes, breaking up the beef as it cooks. Add 1 cup diced onion and 1 tablespoon minced garlic to the skillet and cook for an additional minute.
Add 2 cups uncooked elbow macaroni noodles, 1 (10-ounce) can diced tomatoes, 1 cup milk, and 1 tablespoon salted butter to the skillet and stir to combine. Add 1 cup frozen peas and 1 cup frozen corn and bring the mixture to a simmer.
Once simmering, gradually stir in 1 ½ cups of the 3 ½ cups shredded cheddar cheese and reserve the remaining cheese for the top. Season with salt and pepper to taste.
Transfer the mixture to the prepared baking dish and top with the remaining cheddar cheese. Cover with foil and bake for 30-35 minutes, until the cheese is melted and bubbling. Let the casserole sit for 5 minutes before serving.
Notes
Storage Instructions
In the Refrigerator: Store leftovers in an airtight container. Keeps fresh for up to 5 days.
In the Freezer: Freeze the fully cooked casserole for up to 3 months.
Reheating:
From the fridge: Reheat in the microwave in 30-second increments until heated through.
From Frozen: Thaw overnight in the refrigerator. Bake at 375ºF for 30–40 minutes, or until the center reaches 165ºF.