Peppery kohlrabi slaw

Name: Peppery kohlrabi slaw
Ingredients: 2 whole kohlrabi, peeled; head of spring greens, finely shredded; 2 bunches spring onions, very finely shredded; small bunch French breakfast radishes, quartered, leaves attached; juice 0.5 lemon; 175g mayonnaise; 50g grated horseradish, fresh or from a jar; 2 heaped tsp cracked black pepper