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 1 (7.25-ounce) box macaroni and cheese, along with the cheese packet, 1 (10-ounce) can diced tomatoes, 1 ¼ cup milk, and 3 tablespoons 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. Season with salt and pepper to taste.
Transfer the mixture to the prepared baking dish and top with 2 cups shredded 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.
Reheat in the microwave in 30-second increments until heated through.
In the Freezer
Freeze the fully cooked casserole for up to 3 months.
Thaw overnight in the refrigerator.
Bake at 375ºF for 30–40 minutes, or until the center reaches 165ºF.