Clean the kohlrabi, carrots and celeriac and cut into julienne vegetables. Clean the leek and onions and cut into fine rings.
Put the vegetables together in an appropriately large bowl and mix, seasoning with salt and pepper (if you like it spicy, you can also use some chilli flakes to season).
Cut an appropriately sized piece of roasting tube and seal one side.
Preheat the oven to 175 ° C, do not leave the rack in the oven while it is preheating (it is essential to follow the instructions on the roasting tube package).
Spread the vegetables in the roasting tube. Spread 2 tablespoons of the butter on top in flakes.
If necessary, wash the kitchen-ready fish again and pat dry well, if necessary cut into portion sizes. Drizzle with lemon / lime juice, season with salt and pepper. Place the fillet on the vegetables and spread the rest of the butter on top as flakes. Close the roasting tube. Carefully pierce a few small holes in the top so that the steam can escape while cooking. Place the roasting tube on the cold grid and cook on the 2nd rail from the bottom in the oven at 175 ° C for 30 minutes.
Tip: I always open the roasting tube in a large baking dish!