Cook the rice in plenty of boiling salted water for about 20 minutes. Wash the fish, dab dry, cut into large pieces and sprinkle with lemon juice. Heat oil in a pan. Fry the fish and prawns all around for about 3 minutes.
Season with salt and pepper and remove. Pour stock and cream into the frying fat and bring to the boil. Stir in sauce thickener and bring to the boil. Add fish and prawns to the sauce and simmer for about 3 minutes at low heat.
Season fish ragout with vermouth and lime juice. Wash the dill, put something aside for garnishing. Chop the remaining dill finely and fold into the ragout. Pour rice on a sieve and drain.
Arrange the fish ragout with the rice on plates. Serve garnished with dill and lime.