Cut the fish. Peel and cut the leek and celeriac. Cut the parsley in large pieces.
Simmer the fish and vegetables in water for 15 minutes.
Pour away most of the water, but not all.
Add the oil.
Puree or mash the fish to the consistency you like
Serve the fish with potatoes.
Note: If you have fish allergy in your family, consult a pediatrician before serving fish to you baby!