Heat oil in a large saucepan over medium heat. Add onion and celery; sauté 4 to 5 minutes or until vegetables are tender. Add wine; cook over medium-high heat 1 to 2 minutes or until most of liquid is absorbed. Add rice, broth, water, and 1/4 teaspoon salt. Bring to a boil; cover, reduce heat, and simmer 15 minutes or until rice is tender. Stir in cheese, walnuts, parsley, navy beans, remaining 1/4 teaspoon salt, and pepper.
Divide rice mixture among 4 (6-ounce) custard cups coated with cooking spray. Place a serving plate upside down on top of each cup; invert timbales onto plates. Spoon Two-Mushroom Sauce over timbales. Garnish with parsley sprigs, if desired.