In a medium bowl, combine all the burger ingredients and mix until just combined. Be careful not to overwork the meat.

Divide the mixture into six equal portions and shape each into a patty about 4 inches wide with 1-inch-thick sides. Create a slight indentation in the center of each patty. Cover and refrigerate until ready to cook.

Preheat the cast iron grill pan to medium-high heat. Lightly oil the grill grates, then place the patties on the grill with the indented side facing up. Cook for about 5 minutes until grill marks appear on the bottom.

Flip the patties and continue grilling for another 3 minutes, or until they are slightly firm for medium doneness.

In the last minute of cooking, add 1-2 slices of cheese on top of each patty and cover the grill to allow the cheese to melt. Remove the patties from the grill and toast the buns if desired.

Assemble the burgers with your preferred toppings and serve.