If you make your own hamburger buns start by baking them.
Wash the beets (and dice them if you want them to get ready faster) and cooking them until soft.
Meanwhile preheat the oven to 175 degrees Celsius.
Combine the ingredients for the beef patties in a bowl and form 4 equally sized patties.
Preheat a nonstick pan over medium heat.
Fry the patties for 2 minutes on each side until they start to get golden brown.
Transfer the patties to a oven safe dish, add the crumbled feta cheese and bake in the oven for 7-10 minutes more, until they are as done as you prefer them to be.
Meanwhile peel the cooked beets, and dice them finely.
Slice the red onion and dice the apple (if using).
Add beets, red onion and apple to a bowl and combine with creme fraiche, yogurt or quarck. Add the dijon mustard according to tast and season with salt and pepper.
When the patties are ready, put them on the hamburger buns and top with the beetroot salad. Serve warm!