The salmon should be allowed to come to room temperature, then reheat in the oven at 180C or 360F for approximately 13 minutes. It needs to warm all through. If you insist, it can be rewarmed in the microwave in no time, but I do not recommend it as the pastry will become soggy and of course, the fish will dry out which is exactly the opposite of what we want this dish to be like. Crispy on the outside and moist inside. I would only consider the oven but yes, as a last resort, the microwave will help.