Using a sharp knife, or preferably a mandolin, very thinly slice the kohlrabi, then cut into strips. Tip into a bowl and toss through the spring greens, spring onions and 1-2 tsp sea salt. Set aside for 30 mins and let the salt draw out any excess liquid from the vegetables.
Meanwhile, mix together the mayonnaise, horseradish and black pepper. Give the vegetables a little rinse and squeeze out any excess water with a clean cloth, then tip into a bowl. Add the radishes, dressing and lemon juice, and toss everything together.