Drain cherries, reserving 1/2 cup liquid. Transfer cherries to a medium saucepan. Add reserved cherry liquid, the 1/2 cup sugar, and the tapioca. Cook and stir over medium heat until mixture boils; reduce heat. Simmer, uncovered, for 5 minutes, stirring constantly; keep warm.
In a medium mixing bowl beat egg whites, cream of tartar, and salt with an electric mixer on medium speed until stiff peaks form (tips stand straight); set aside. In a small mixing bowl beat egg yolks for 2 to 3 minutes or until thick and lemon colored; add the 1/3 cup sugar. Beat 1 minute more. Stir a small amount of egg white mixture into egg yolk mixture to lighten. Fold remaining egg yolk mixture into egg white mixture. Sprinkle flour over egg mixture; fold in.
Pour hot cherry mixture into six 6- to 8-ounce oven-safe mugs, casseroles, or custard cups or one 1-1/2-quart casserole. Pour batter over cherry mixture. Bake in a 325 degree F oven about 30 minutes for the small cups or casseroles or 35 to 40 minutes for the 1-1/2-quart casserole or until top springs back when lightly touched. Serve warm. Makes 6 servings.