Line a 10-inch round cake pan with plastic wrap.

In a bowl, combine ground beef, chopped yellow onion, steak sauce, Worcestershire sauce, salt, and ground black pepper. Press the mixture firmly into the prepared cake pan to shape a large beef patty. Cover the pan with plastic wrap and refrigerate until the patty is firm, about 2 hours.

Preheat an outdoor grill to medium-high heat and lightly oil the grill grate.

Lightly oil a pizza peel. Remove the plastic wrap from the top of the beef patty, then invert the cake pan onto the pizza peel to release the patty. Remove any remaining plastic wrap. Using a grill spatula and a gentle shaking motion, carefully slide the patty from the pizza peel onto the preheated grill.

Grill the beef patty until browned on one side, about 7 to 10 minutes. Use the grill spatula and pizza peel to carefully flip the patty. Arrange the Cheddar cheese slices and cooked bacon strips on top of the burger. Continue cooking until the burger is firm and no longer pink in the center, approximately 10 minutes more. Confirm doneness with an instant-read thermometer inserted into the center, which should read at least 160 degrees F (70 degrees C).

Place the bottom half of the Mardi Gras King Cake on a serving platter. Set the cooked burger on top of the bottom cake half, then cover with the top half of the king cake. Spread the reserved icing over the top cake half and sprinkle the purple, yellow, and green decorating sugars evenly over the icing.