1. Preheat oven to 350 degrees. Coat two 9-by-2-inch round cake pans with cooking spray; set aside. Melt butter in a medium skillet over medium heat. Add onion, garlic, and celery; cook until softened, about 7 minutes. Transfer to a large bowl; let cool.

2. Add beef, carrot, tomato sauce, Worcestershire sauce, eggs, oats, ground mustard, salt, and pepper to bowl with vegetable mixture; mix thoroughly with your hands.

3. Gently press into prepared pans. Bake until an instant-read thermometer registers 160 degrees, about 25 minutes. Let stand 5 minutes; carefully remove with a spatula, and transfer to a wire rack set over a rimmed baking sheet. Tent with foil.

4. Bring 2 cups water to a boil in a small pan. Add reserved carrot peels; cook until tender, about 1 minute. Using a slotted spoon, transfer to paper towels to drain.

5. Place 1 meat loaf on platter; spread 1 cup warm mashed potatoes over top. Sprinkle with 3 tablespoons peas. Top with remaining meat loaf. Frost top and sides with 3 cups mashed potatoes. Fill piping bag fitted with a 1/2-inch star tip (such as Ateco No. 825) with remaining mashed potatoes. Pipe borders around cake. Using tip of a 1/4-inch plain round tip (such as Ateco No. 10), punch out circles from carrot peels. Decorate cake with carrot circles and remaining peas. Serve immediately.