Put the pork into a large bowl – it is easier to work with if it is not too cold from the fridge. Grate the apple and the onion together (this can be done in the food processor). Add to the pork in the bowl. Add the bread crumbs to the mixture and work together with clean hands until everything is well blended.
Finely chop the sage and add to the mixture with a pinch of salt and a grind of pepper, and continue working until completely mixed.
Divide the mixture into eight equal portions and form into patties. Refrigerate until ready to use, or wrap each tightly and freeze for up to 3 months.
Lightly salt and pepper the patties.
Put patties on the grill and 20 min or until cooked thoroughly.
Cut up second onion, and caramelize them in vegetable oil.
While the burgers are cooking, put butter in a small pan. Put it on medium heat until it starts to bubble. Turn down to low heat, chop the sage and put it in the butter. Cook for 5 minute Dip buns in the browned butter and place on grill to toast.
Remove buns and burgers and put it all together and top with caramelized onions.