In a medium saucepan cook the rice with the water.
Preheat oven to 450 degrees F (225 degrees C).
Line a pie plate with pastry 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 pie shell. 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 pastry. Seal edges and pierce top.
Bake at 450 degrees F (225 degrees C) for 10 minutes. Reduce heat to 350 degrees F (175 degrees C) and continue to bake for another 30 to 35 minutes.