Rinse the pangasius fillets and pat dry (frozen fillets also work), season with salt and pepper. Clean the leek and cut into very thin rings.
In the meantime, heat the oil and briefly steam the leek in it.
Cut the washed tomatoes into cubes and mix with the finely chopped parsley. Also cut the onion into very thin slices.
Spread the leek on 4 pieces of aluminum foil, place a pangasius fillet on each. Spread the diced tomatoes and the onion rings on top. Close the aluminum foil so that no liquid can escape.
Place the parcels on a baking sheet and cook in the oven preheated to 180 ° C for 20-25 minutes (approx. 3 minutes more for frozen fish).