Place spinach in a glass pie plate along with 3 tablespoons water. Cover with plastic wrap and microwave on HIGH power 3 minutes. Uncover and drain carefully. Coat a 1-1/2-quart baking dish with nonstick cooking spray.
Heat oven to 350 degrees F. In a large bowl, stir together the rice, spinach and 1/4 teaspoon of the salt.
In a small bowl or measuring cup, whisk together the sour cream, egg, 2 tablespoons of the Parmesan, mustard, remaining 1/4 teaspoon salt and black pepper.
Set aside 2 tablespoons of the sauce. Combine remaining sauce (a generous 3/4 cup) with the rice-spinach mixture. Stir until well coated. Transfer to the prepared casserole, spreading level.
Place salmon, skin side up, on a cutting board. Peel off skin, and flip over. Remove any bones with tweezers. Slice crosswise at an angle into 1/2 inch thick slices. Fan slices over the rice mixture.
Thin reserved sauce with 1/2 teaspoon water. Drizzle over salmon slices. Top with remaining tablespoon Parmesan cheese. Bake in 350° oven for 25 to 30 minutes, or until casserole registers 150 degrees F on an instant read thermometer. Let stand 5 minutes at room temperature before serving. Makes 4 servings.