Fry apple, celery and onion in hot oil over med–heat until mixture is tender (6–8 min); then leave it to cool down to room temperature.
Grab a large bowl and pop the apple mixture, ketchup, and all the seasoning. Add the meat (bison/beef) and mix well. Divide into six 3⁄4ʺ thick burger patties.
Charcoal: Grill patties on a well–oiled rack of an open grill directly over med–hot coals until no longer pink; internal temp 160°F (16–18 min). Turning once, halfway.
Gas grill: Preheat and drop the heat to med–hot. Grill as above.
Just before removing the meat, toast the bread on both sides. Brush with melted butter when you’re done.
Top the toast with lettuce, patties and melon relish.
Combine the chopped melon, strawberries, onion, sweet pepper, cilantro, fresh mint, lemon/lime zest and juice. Toss lightly to coat everything in the juice.
Leave it to sit at room temperature for half an hour or so, stirring now and then. This way the flavors will combine. Enjoy!