Preheat your grill to medium-high. Place the first 8 ingredients in a bowl and gently mix to combine. Moisten your hands with cold water. Form the meat into four 3/4 inch thick patties. Grill the burgers 4 minutes per side, or until entirely cooked through and the centre of each burger reaches 160 degrees F (71 degrees C) on an instant read meat thermometer. Turn the heat to low. Top each burger with slices of the cheese and 2 pieces of bacon. Spread the buns with mayonnaise. Top the bottom buns with the spinach. Set on the patties and onion and serve.
Recipe Options: If you don't care for lamb, make these burgers with ground beef. Instead of Irish cheddar, use another tangy and flavourful cheese, such as Old Canadian cheddar. Accent the burgers with other toppings, such as mustard, pickles and tomatoes.