Preheat the oven to 180 °C.
Remove the skin from the onions and cut them into one and a half cm leaves. Clean and rinse the celery and fennel; set the fennel greens aside. Cut the celery and fennel into two-inch pieces.
Rinse and dry the whole lotus and remove all skins with a sharp kitchen knife. Sauté the fish in a roasting pan in the oil over medium heat until browned all over and remove.
Sauté the fennel and mustard seeds in the oil, add the prepared vegetables and sauté until translucent, season lightly with salt and pepper.
Form the vegetables into a gratin dish and place the lotte on top. Spread the unpeeled garlic, thyme and bay spice evenly around the fish and cook the fish in the heated oven on the middle shelf for twenty-five to thirty minutes, brushing a few times with butter.
In the meantime: peel the potatoes and cut them into ten to fifteen mm cubes. Soften the potato cubes in the vegetable soup with the bay leaf spice, chili pepper and unpeeled garlic at medium temperature. Drain into a colander, reserving the stock. Roast the potato cubes in a frying pan in the olive oil at low temperature, season with salt and cayenne pepper.
To finish, remove the lotte from the gratin dish, keep warm. Pour some stock to the vegetables. Put in the fennel greens and lemon zest, let both steep for a few min and