Cook the potatoes as usual (boiled potatoes or jacket potatoes).
Cook the vegetables in the vegetable stock and then drain, collecting the stock.
Melt the margarine in a saucepan. Add the flour, stir well and pour the cream and the vegetable stock on top. Cut the fish into cubes, add to the thickened vegetable stock and let it steep. Add the vegetables again. Season with salt and pepper and finally add parsley and chives.
Arrange the potatoes with the fish and vegetable ragout on plates and serve immediately.