Gently form the ground chicken into four oval 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 the patties to prevent them from doming as they cook. Refrigerate the patties until ready to grill.
In a small bowl mix the mayonnaise and mustard.
Prepare the grill for direct cooking over medium heat (180°C to 230°C).
Lightly coat the onion slices on both sides with oil, season with salt and pepper, and then grill over direct medium heat, with the lid closed, until tender, 6 to 8 minutes, turning once. Remove from the grill.
Lightly coat the onion slices on both sides with oil, season with salt and pepper, and then grill over medium heat, with the lid closed, until tender, 6 to 8 minutes, turning once. Remove from the grill.
Lightly brush the patties on both sides with oil and season evenly with salt and pepper. Grill over direct medium heat, with the lid closed, until fully cooked but still juicy, 9 to 11 minutes, turning once. During the last minute of grilling time, place a slice of cheese on top of each patty to melt. Remove from the grill.
Lightly brush the patties on both sides with oil and season evenly with salt and pepper. Grill over medium heat, with the lid closed, until fully cooked but still juicy, 9 to 11 minutes, turning once. During the last minute of grilling time, place a slice of cheese on top of each patty to melt. Remove from the grill.
Brush the bread on one side with oil and toast the slices over direct medium heat until browned on both sides, about 1 minute, turning once. Spread the rye with the mayonnaise mixture. Build each sandwich with a patty and onions. Serve warm.
Brush the bread on one side with oil and toast the slices over medium heat until browned on both sides, about 1 minute, turning once. Spread the rye with the mayonnaise mixture. Build each sandwich with a patty and onions. Serve warm.
