Cut out a few circles from the fillo dough (diameter 7.5 cm) and separate the individual dough sheets from each other. (Fillo dough consists of very thin dough sheets.) Brush 24 dough circles with butter, press 2 on each other (with the butter on the inside) and bake in the kitchen oven at 180 °C for 6 min. Clean the vegetables, chop the onion finely and cut the remaining vegetables into narrow strips (julienne). Sauté the finely chopped onion and a clove of garlic in a little oil, almost at the same time add the celeriac, after 2 min the carrot and after 5-6 min the zucchini strips. Remove from the heat when the vegetables are cooked but still crunchy. Add salt and pepper and spread the vegetables evenly on the baked pastry circles. Drape the fish slices in the shape of roses on a tray lined with parchment paper, pour a little oil over them, sprinkle with salt and cook for 1-2 min at 200 °C in the kitchen oven. Remove from the stove and garnish the vegetables with it.
Tip: The flesh of young, small zucchini is much more tender than that of larger ones.