How to Make It
Preheat oven to 350°F. Toss together blackberries, lemon zest, lemon juice, cornstarch, and 1/4 cup of the sugar in a large bowl. Transfer to an 11- x 7-inch (2 quart) baking dish.
Whisk together flour, oats, brown sugar, salt, cinnamon, and remaining 3 tablespoons sugar in a small bowl. Using a pastry cutter or your fingers, work butter into flour mixture until almost combined. Drizzle canola oil over mixture, and continue working mixture until there are no large clumps. Sprinkle evenly over blackberries.
Bake in preheated oven until top is golden and blackberries are bubbling, about 40 minutes. Remove from oven, and let rest 10 minutes before serving.