Let the salmon thaw, rinse and pat dry. Season all around with salt and pepper. Grease a casserole dish and place the fish side by side. Mix sour cream, cream and champagne and bring to the boil carefully.
Season to taste with salt, pepper and stock. Clean and wash spring onions and cut into thin rings. Spread on and around the fish. Pour sauce over it. Cook in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 20-25 minutes.
Meanwhile, cook the rice in boiling salted water according to the instructions on the packet. Arrange rice and fish with sauce and lemon wedges.