Preheat oven to 350 degrees F (175 degrees C). Lightly butter one 9x13 inch baking dish.
In a cup, combine salad dressing mix, water and lemon juice.
Arrange salmon fillets in a single layer in the prepared baking dish. Pour the water mixture over the top and place the sliced mushrooms over the salmon.
Bake, covered, for 15 minutes. Remove cover and bake for an additional 15 minutes, basting with cooking liquids.
