To Make the Pesto:

1. Preheat oven to 320°F. Roast hazelnuts for about 9 minutes. Remove, allow to cool, and put back in the oven for another 9 minutes.
2. Wash the leaves for the pesto and blanch in lightly salted water for 1 minute. Drain, then cool in ice water and squeeze out excess moisture.
3. Add leaves, hazelnuts, lemon juice, salt, and a few ice cubes. Blend, then add olive oil to achieve desired consistency. Serve warm with stuffed kohlrabi.

To Make the Stuffed Kohlrabi:

1. Remove leaves and set aside. Clean the vegetables with a vegetable brush under running water, then remove and protruding parts with a vegetable peeler. Boil in lightly salted water until tender (but not too soft because they'll cook in the oven), then drain.
2. Remove leaves from celery, peel the carrots, and cook both in lightly salted water or a few minutes, then remove.
3. Peel potatoes and cook in the same water or steam until soft. Add potatoes to a bowl and mash. Finely chop the carrots, celery, and parsley, then mix with the potatoes.
4. Cut the cap off the kohlrabi and using a melon baller or spoon, remove the pulp (save to eat later). Salt the insides, then flip them upside down and let sit for a few minutes, to draw out more water. Preheat oven to 355°F.
5. When ready, fill each kohlrabi with the potato mixture and sprinkle with bread crumbs. Grease a baking dish. Add stuffed vegetables (with the caps on) and season with olive oil to taste.
6. Fill each kohlrabi with the compound previously prepared and sprinkle the surface with breadcrumbs. Bake in preheated oven until golden, about 20 minutes.