Heat oven to 400 degrees F.
Combine berries, rhubarb, sugar, lemon juice and cornstarch in saucepan. Cook over medium-high heat 10 minutes. Reduce heat to medium; cook 2 minutes, until liquid is thickened, clear and bubbly. Let cool slightly.
Open pastry sheets on work surface. Cut each sheet into 4 equal pieces. Coat 8 standard-size muffin cups with nonstick cooking spray. Gently press middle of 1 pastry square into bottom of muffin cup. Fill with almost 1/4 cup of filling. Fold over 4 pastry flaps, one on top of the other, covering the filling; brush with water to seal. Repeat with remaining puff pastry and filling.
Bake in 400 degree F oven 15 to 20 minutes, until tops are puffed and golden brown. Cool in pan on rack 10 minutes. Remove from muffin cups to rack to cool 5 to 10 minutes. Dust with confectioners sugar. Makes 8 servings.