Preheat your Oklahoma Joe’s Rider 900 DLX to 450 degrees, adding the griddle to the Flex Grate system and making sure the heat baffle is seat to “Sear.”
When grill is preheated, add the butter, allowing it to melt, followed by the sliced onion, stirring to coat. Cook onions, stirring occasionally, until they have softened and become caramelized, about 7 minutes.
While the onions cook, add the ground venison, salt, pepper, Worcestershire sauce, and melted lard to a large mixing bowl and gently combine with clean hands, being careful not to overmix.
Divide the meat into four equal portions and form into rectangular patties (so they are roughly the same shape and size as the rye bread) and set aside.
When onions have caramelized, move over to one side of the griddle to stay warm. Add the burger patties to the open space of the griddle and cook for 2 minutes, per side, or to desired doneness. Remove immediately and begin the shutdown process for the Rider.
To assemble the patty melts: Place 1 slice of rye bread on your prep surface, followed by a slice of Swiss cheese, ¼ of the caramelize onions, cooked burger patty, a slice of American cheese, and top with another slice of rye bread. Repeat with remaining three sandwiches.
Butter the top slices of bread and place the sandwiches, buttered side down, on the still-hot griddle (carryover heat will help ensure the sandwiches get toasted!).
Carefully butter the slices of bread facing up before flipping the sandwiches to toast the other sides until beautifully golden brown.
Remove from grill and enjoy!