Peel the kohlrabi and cut into 2 cm (¾ inch) dice. Place in a saucepan and add enough milk to cover. Bring to the boil, then reduce the heat to low and simmer until cooked through. Strain, reserving both the solids and milk. Transfer the kohlrabi to an upright blender and blend, adding just enough reserved milk to form a thick, smooth purée. Season with salt and pepper. Set aside and keep warm.