For the potato and kohlrabi rösti, first mix potatoes and kohlrabi and mix with egg and flour and season with salt, pepper and chili to taste. Using a tablespoon, place the mixture in hot oil and press flat. Fry out on both sides and pat dry with kitchen paper.
For the onion mustard, finely chop the onion, mix with mustard and curry and refrigerate. Serve the potato and kohlrabi rösti with the mustard.