Juice and zest the lemon.
Trim, peel and mince 1 garlic clove and the shallot.
Rinse, dry and pick leaves from the parsley to equal 1/4 cup. Mince and reserve.
In a large mixing bowl combine 1/3 cup mayonnaise, 2 Tbsp lemon juice, 1 tsp lemon zest, 2 tsp minced garlic, minced shallot, chopped parsley, 1 Tbsp salt and 1 tsp pepper, 1 tsp dried mustard and a pinch saffron.
Peel, trim and shred the kohlrabi using a hand grater or food processor. Transfer to a strainer, add 1 Tbsp salt, toss well and let drain for 10 minutes. Rinse and pat dry and add to the mixing bowl, toss well with the mayonnaise and set aside.
Place the ground beef in a bowl. Season with 1 tsp salt and 1/4 tsp pepper and mix well. Form 4 patties.
Cut the buns in half and spread each half with some butter.