Place the salmon fillets in a microwave-safe dish and pour over the lactose free milk. Cover the dish with cling film, ensuring it does not touch the food and leave a small area open.
Cook in the microwave on a full power for three minutes, then remove and check the fish. Turn the fish so that the cooked parts are in the centre of the dish. Cook on full power for another one minute until pale pink and cooked. Leave to stand.
Put the lactose free butter into the microwave-safe jug and place in the microwave. Cook on full power for 30 seconds. Remove the jug from the microwave, tip in the rice flour and whisk until it is blended thoroughly.
Carefully pull back the cling film from the microwave-safe dish and remove two tablespoons of the lactose free milk. Add it to the rice flour and lactose free butter and whisk to form a smooth paste with no lumps.
Continue to add the lactose free milk, a few spoonfuls at a time, and whisk until all the milk has been added. Keep covering the salmon with the cling film to keep it warm.
Place the jug into the microwave and cook on full power for 3-4 minutes, removing the jug to whisk thoroughly at one minute intervals. This will ensure there are no lumps.
After 3–4 minutes, the sauce should boil, but be careful it does not spill over the sides of the jug. Once boiled, carefully remove the jug from the microwave and whisk thoroughly. Add the lemon juice, saffron and a grind of black pepper (if using) – and whisk.
Either pour the sauce over the fish or, using a fish slice, remove the fish to individual plates and spoon over the sauce. Sprinkle with chives if using.