PREHEAT OVEN to 350°F (180°C). Line a 12 x 16-inch (30 x 40-cm) jelly roll pan or cookie sheet with parchment. Set aside. Keep a clean, damp dishtowel on hand.
PLACE THE CHOPPED APRICOTS and berries into a medium bowl. Add the sugar and cornstarch and toss lightly. Set aside.
PLACE A LARGE PIECE OF PARCHMENT paper on the counter. Take the filo out of its package and unroll. Separate one sheet and place on top of the parchment. Spray with the oil. Place a second sheet on top and spray again. Repeat with two more sheets. Cover the remaining filo with a piece of parchment or waxed paper and top with a clean, damp dishtowel.
PLACE HALF THE FILLING along the long end of the filo, 2 inches (5cm) from the edge. Fold the right and left sides (the short sides) in 1 inch (2.5cm). Roll the long end of the filo (with the filling), into a tight, long log. Place it on the baking sheet. Repeat to make another log.
BAKE THE STRUDEL for 40 minutes, or until it’s lightly browned on top. Let the strudel cool and cut it into 2-inch (5cm) slices. Serve warm or at room temperature. Store covered at room temperature for up to two days. Reheat to serve.