Combine all ingredients, except baps, in a bowl. Form the mixture into 4, 1-inch thick patties. Grill over medium heat 4 to 5 minutes per side, or until entirely cooked through and centre registers 160 degrees F or above on an instant read meat thermometer. Sandwich the patties in the Scotch baps with the lettuce, tomato and onion. Flavour with other ingredients, as desired.