Wash the potatoes and cook covered for about 20 minutes. Peel, wash and cut the kohlrabi into thin slices. Peel and wash the carrots, cut them in half lengthwise and cut them into 4-5 cm long pieces. Cook both in 1⁄2 l boiling salted water covered for about 8 minutes.
Wash the asparagus, cut off the woody ends generously. Cut asparagus into pieces and add about 3 minutes before the end of cooking time. Drain the vegetables, collecting 1⁄4 l vegetable water. Drain potatoes, rinse and peel.
Wash parsley, shake dry and pluck 8-12 leaves. Wash the fillets, dab dry and season with pepper and a little salt. Wrap 2 slices of bacon around each fillet, wrapping 2-3 parsley leaves in each.
Heat butter. Sweat flour in it. Stir in cream and vegetable water. Bring to the boil and simmer for about 3 minutes. Season to taste with salt, pepper and nutmeg. Grate cheese roughly. Cut potatoes into slices.
Alternate with the vegetables in a very large casserole dish, leaving a depression in the middle. Pour the sauce evenly over it, except for 5 tablespoons. Sprinkle with cheese. In a preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see
manufacturer) bake for about 25 minutes.
Heat 2 tablespoons of oil in a frying pan. Fry the fillets for about 10 minutes. Cut into thick slices and place in the middle of the gratin about 10 minutes before the end of the baking time. Drizzle with the rest of the sauce.
Clean, wash and shake the rocket dry and chop finely together with the rest of the parsley. Mix with lemon zest and 5-7 tbsp. oil. Season to taste with salt and pepper. Remove the gratin and sprinkle with some rocket oil, add the rest.
Drink tip: dry white wine, for example a Silvaner from Franconia