Bring water to boil in a 1 quart saucepan, stir in salt and rice, cover, reduce heat and simmer for 20 minutes. Remove from heat, fluff rice gently with a fork; set aside.
Preheat oven to 400°F. Grease a 13 x 9-inch baking dish.
In a skillet heat butter over medium heat; saute onion until tender. Add flour and dry mustard, stirring until smooth. Cook for 1 minute, stirring constantly.
Gradually add milk and reserved salmon liquid. Cook over medium heat, stirring constantly, until thickened and bubbly. Add 1 cup of the cheese and cayenne pepper; stir to melt cheese.
Spread spinach in baking dish. Layer salmon and rice and top with cheese sauce. Bake, uncovered, for 15 minutes.
Sprinkle with remaining 1/2 cup cheese and bake an additional 5 minutes until cheese melts.