Melt the fat in a pot. Stir in flour, sweat for 1-2 minutes. Slowly stir in the cream, milk and stock and simmer for 6-8 minutes, stirring occasionally. Add vegetables. Simmer for about 10 minutes, stirring occasionally. Season to taste with salt, pepper and lemon juice.
Meanwhile, cook the rice in boiling salted water according to the instructions on the packet. Wash the fish, dab dry and cut into cubes. Peel and finely dice onions. Heat oil in a pan. Fry the fish for about 5 minutes while turning.
After about 2 minutes add the onions. Season with salt and pepper.
Drain the rice. Add fish and rice to the vegetables. Wash chives, shake dry and cut into fine rolls. Arrange fricassee on plates, sprinkle with chives.