Birthday Meatloaf Cake

Name: Birthday Meatloaf Cake
Ingredients: Vegetable-oil cooking spray; 1 tablespoon unsalted butter; 1 medium onion, finely chopped (about 1 cup); 2 garlic cloves, minced; 1 (about ½ cup) celery stalk, finely chopped; 2 ½ pounds ground beef; 1 small carrot, peeled and finely grated (about ½ cup), peels reserved for decorating; ¼ cup tomato sauce; 1 tablespoon Worcestershire sauce; 1 large egg, lightly beaten; ½ cup old-fashioned rolled oats; 1 tablespoon ground mustard; 1 ½ teaspoons salt; ¼ teaspoon freshly ground pepper; Mashed Yukon Gold Potatoes; ¼ cup cooked green peas