Clean, wash and cut the leek into rings. Peel and wash the carrots, cut them in half lengthwise and cut them into slices. Peel and finely chop the onion. Heat butter in a large pot. Sauté the vegetables for about 5 minutes.
Cut 4 strips from parchment. Preheat oven (electric cooker: 175°C/circulating air: 150°C/gas: see manufacturer). Rinse fish and pat dry. Place fish fillets one by one on the paper strips, twist together at the ends, spread curry tomato sauce on the fish fillets.
Sprinkle with sesame seeds. Close the parchment strips to form packets. Bake in the oven for about 15 minutes. Rice tastes good with it