Melt butter in a small saucepan over medium heat. Add onion; cook 3 minutes, stirring frequently. Stir in water, rice, and 1/4 teaspoon salt; bring to a boil. Cover, reduce heat, and simmer 40 minutes or until liquid is absorbed. Remove from heat; stir in 1/4 teaspoon salt, parsley, chives, juice, and oil. Sprinkle each serving with walnuts.