Finely grate one of the onions and put it in a bowl with the mince, garlic, horseradish and season with salt and pepper. Mix well.
Shape into 4 equal size burgers, wrap each burger with 2 rashers of bacon and chill for 30 minutes.
Preheat grill to medium-high. Slice remaining onions, heat oil in a frying pan, add the onions and fry over a medium heat for 8 minutes, stirring frequently until onions are golden brown. Drain them on kitchen towel and keep warm.
Cook the burgers under the grill for 3-5 minutes on each side or until cooked to your liking. Lightly toast the rolls and arrange the lettuce on the base.
Add the cooked burgers, topped with some fried onions, a spoonful of relish and then top with cheese. Grill them again for 2 minutes until cheese melting. Then add the top of the roll.
for an extra kick add mustard on top of the burger before adding the cheese