Preparation :
Put the sliced onions and 1/4 glass of olive oil into a saucepan and begin to saute. After 5 minutes add 3 glasses water and cook for 20 minutes until the onions become like a puree and leave aside.
Peel the potatoes, put them in a bowl with the celeriac, carrots cut into two lengthways, adn then the garlic chopped small, and the tomatoes (chopped into small pieces). Spread this over the prepared onion mixture, cover and cook for 20 minutes over medium heat. After gutting and cleaning the fish, slice it into circles that are 5 cm thick. Add the fish to the vegetable mixture with the 1/2 tablespoon salt and the rest of the olive oil. After cooking for 15 minutes over a fairly high heat, add lemon juice and parsley. Remove the pan from the oven and serve after it has cooled down.