Heat oven to 225°F. Grease 2 cookie sheets with shortening or cooking spray. In large bowl, beat egg whites and cream of tartar with electric mixer on medium speed until soft peaks form. On high speed, gradually beat in 1/2 cup sugar until stiff glossy peaks form and sugar is almost dissolved. Spoon meringue into 8 (4- to 5-inch) rounds on cookie sheets, hollowing center slightly with back of spoon to form shells.
Bake 1 hour. DO NOT OPEN OVEN. Turn oven off; let meringue shells stand in oven with door closed 1 hour.
Meanwhile, in 2-quart saucepan, mix all filling ingredients except strawberries. Cook over medium-low heat 15 to 20 minutes, stirring frequently, until filling is slightly thickened and rhubarb is soft. Stir in strawberries. Cool completely, about 1 hour.
Spoon about 1/4 cup filling into each meringue shell. Store filling in refrigerator.