Using a sharp knife, mince the frozen bacon (a food processor would overprocess it). Mix all the patty ingredients. Then gently form six patties of equal size, each about 2 centimetres thick. With your thumb or the back of a spoon, make a shallow indentation about 2.5 centimetres wide in the centre of each patty to prevent them from forming a dome as they cook. Refrigerate the patties until ready to grill.
Prepare the grill for direct cooking over medium–high heat (200°C to 230°C).
Lightly brush the patties with oil and then grill over direct medium–high heat, with the lid closed, until cooked to medium doneness (70°C), 8 to 10 minutes, turning once. During the last 30 seconds to 1 minute of grilling time, top each patty with a slice of cheese to melt and toast the buns, cut-side down, over direct heat.
Spread the bottom bun halves with mayonnaise and the top halves with mustard. Build each burger with a lettuce leaf, two tomato slices, a patty, three pickle chips and an onion slice. Serve warm.