In a medium saucepan cook the rice with the water.
Preheat oven to 230 C / Gas 8.
Line a 23cm pie dish with one sheet of pastry, trim the edges and set aside.
In a large saucepan over medium heat, saute the onions and mushrooms in the butter until soft.
Combine the cooked rice with the mushroom soup and spread half of the mixture over the bottom of the pastry case. Spread the mashed salmon over the rice mixture. Top with the sauteed mushroom/onion mixture and then top with the remaining rice mixture. Sprinkle with grated cheese and cover with the second sheet of pastry. Trim edges, seal and pierce top.
Bake at 230 C / Gas 8 for 10 minutes. Reduce heat to 180 C / Gas 4 and continue to bake for another 30 to 35 minutes.