Prepare the onion rings: Peel the onion and slice it into thick rounds, about ½ to ¾ inch wide. Carefully separate the rings, choosing the largest ones for stuffing. Pat them dry with a paper towel to help the coating stick later.
Cook the bacon: In a skillet or oven, cook the bacon until crispy, then crumble it into small pieces. Set aside to cool. The smoky bacon flavor will add depth to the beef mixture inside each onion ring.
Prepare the beef filling: In a mixing bowl, combine the ground beef, shredded cheddar cheese, and bacon crumbles. Season generously with salt and pepper, and mix gently with your hands until evenly distributed. Avoid overmixing to keep the filling tender.
Stuff the onion rings: Place one onion ring flat on a clean surface. Press a small amount of the beef mixture into the ring, packing it gently but firmly to fill the entire space. If the onion rings vary in size, you can stack smaller rings inside larger ones and press the mixture between them to help the structure hold. Repeat until all rings are filled.
