Preheat oven to 350°F. Coat two 8-inch round cake pans with cooking spray. Combine ground beef, bread crumbs, onion, eggs, soup mix, steak sauce, garlic and black pepper in a large bowl; mix until well combined. Divide ground beef mixture in half; press each half gently into a prepared pan.
Bake meat loaves until centers are no longer pink and instant-read thermometer inserted in the centers registers 170°F, 55-60 minutes. Do not turn off oven.
Invert 1 meat loaf onto an ovenproof serving plate. Spread 1⁄2 cup mashed potatoes evenly on top. Place second meat loaf on top. Spread remaining potatoes on side and top to resemble a cake.
Return meat loaf to oven; bake until thoroughly heated, about 10 minutes. Cut flower shapes from bell pepper with mini cookie cutters and place on top of cake, if desired. Cut into wedges to serve.