Boil peas in salted water for 2 minutes, strain, chill cold and drain.
Peel potatoes and cut into 1.5 cm cubes. Heat the oil in a pan and fry the potato cubes for about 10 minutes over medium heat.
Peel kohlrabi and cut julienne. Wash spring onions, cut the light pieces into 2 cm long pieces and the dark pieces into fine rings.
Add kohlrabi and spring onions to the potatoes, season with salt, pepper and cook for about 5 minutes al dente. Stirring often. Add and heat peas.
Plane radishes into very thin slices.
Serve rösti and garnish with the radish slices.