Peel carrots, parsley roots and parsnips. Cut in half lengthwise. Wash celery and cut in half. Also wash leeks, cut in half lengthwise and in half or thirds widthwise (depending on size).
Boil celery and leeks separately in salted water, rinse and dry. Boil or steam the root vegetables al dente. Then brown them in a little butter.
Preheat the oven to 180 °C. Grease a baking dish. Place the root vegetables in the baking dish with the cut side facing up. Place leek and celery in between.
Mix eggs, egg yolk, whipped cream, crème fraîche, lemon juice and zest, salt and pepper. Empty over the vegetables.
Put the root vegetable casserole in the oven for about 20 minutes.