To make the lingonberry mayonnaise, in a small bowl, stir the lingonberry jam and mayonnaise together. Set aside.
For the burgers, in a medium bowl, combine the shaved reindeer meat and ground beef.
Form the meat into 4 equal-sized patties, pressing your thumb into the middle of each to make a little indention.
Season the patties on both sides with salt and pepper.
In a large frying pan, melt about 2 tablespoons butter over high heat.
Add the patties and cook for about 3 minutes per side, flipping once until your ideal doneness.
Place 2 to 3 slices of cheese on each patty during the last minute of cooking and cover with a lid until the cheese melts.
While the patties are cooking, you can fry the bacon until crispy and toast the buns in a separate pan with some butter.
Add a good dollop of the lingonberry mayonnaise on both sides of each bun.
Arrange some baby spinach leaves on the bottoms of the buns then top with a patty, followed by the bacon.
Finish with the bun tops and serve immediately.
