Preheat oven to 400°F. Coat four 12 inch squares of aluminum foil with butter to 1 inch from the edges. Make a bed of spinach on each piece of foil. Place salmon on spinach. Combine cream, yogurt, walnuts, horseradish, sugar and lemon juice. Pour over salmon. Double fold foil to seal packets and place on a baking sheet. Bake for 15 minutes. Check salmon in one packet; fish should be opaque throughout and firm to the touch. Serve at once. 443 calories per serving.