Preheat oven to 180 degrees convection or 200 degrees top/bottom heat. Peel carrots and cut into approx. 3 mm thin slices. Peel kohlrabi, quarter and cut into approx. 5 mm thin slices. Crack walnuts and roughly chop seeds. Heat 6 tbsp oil in a pan, add kohlrabi and carrots and sauté over medium heat for 5 minutes. Add 4 tbsp water and simmer covered for 10 minutes until tender. Season with 2 tsp salt.
In a tall container, blend yogurt or crème fraîche, eggs and Gorgonzola with a blender.
Season to taste with a pinch each of salt and pepper. Drain the cooked vegetables on a plate lined with kitchen paper and let cool briefly. Grease a quiche dish with a diameter of about 26 cm with butter.
Remove the puff pastry from the baking paper and place it in the dish. Cut off the protruding ends. Then spread the vegetables flat on the pastry and cover evenly with the yogurt-Gorgonzola cream. Bake the quiche in the preheated oven on the middle rack for about 25 minutes. After 20 minutes of baking, add the chopped walnuts on top of the quiche and bake for another 5 minutes. The quiche is done baking when the pastry is golden brown and the crème is no longer runny. Divide quiche into portion-sized pieces, arrange on a plate and garnish with garden cress.