On lightly floured surface, roll out half of the pastry and fit into 9-inch (23 cm) pie plate; set aside. In microwaveable measure, microwave rhubarb at High for 50 seconds or until slightly softened. (Or place in steamer and steam for 3 minutes.)
In large bowl, combine rhubarb, apples, blackberries and raspberries. Combine sugar, flour and cornstarch; toss with fruit to coat evenly. Transfer to pie shell; dot with butter and sprinkle with lemon juice.
Roll out remaining pastry; moisten rim of pastry shell and fit pastry over filling, pressing gently along rim. Trim edge and crimp; slash decorative B in centre of top. Bake in 425°F (220°C) oven for 15 minutes; reduce heat to 350°F (180°C) and bake for 35 minutes or until pastry is golden and filling bubbly. Let cool on rack.