Wearing rubber gloves, wash the nettles, shake dry and pick off the leaves.
Blanch the nettle leaves in plenty of boiling salted water. Drain, cool in ice water, then drain again.
Peel the kohlrabi and the sweet potatoes, then halve and thinly slice. Reserve the kohlrabi greens.
Mix the kohlrabi, sweet potatoes and nettles, sprinkle with salt and place in a casserole dish.
Whisk together the eggs and milk. Season with salt, pepper and some freshly grated nutmeg.
Add the milk to the vegetables and bake in a preheated oven at 200°C (180°C fan, gas 6) for 15 minutes.
Meanwhile, blitz the wholegrain bread to crumbs in a blender. Grate the cheese and mix with the breadcrumbs.
Scatter the cheese and bread mixture over the casserole and bake for 40 minutes. Garnish with the kohlrabi greens.