Set a rack in the middle of the oven and preheat the oven to 350°F. Butter an 11x7x2-inch ceramic baking dish.
Toss the peaches, blackberries, flour, cornstarch, vanilla, sugar, lemon juice, and zest in a large bowl. Pour into the prepared dish.
In a separate bowl, stir together the flour, oatmeal, brown sugar, salt, and cinnamon. Add in the butter and rub into the dry ingredients with your fingers until pea-size lumps form. Add in the pecans with your fingers, squeezing to combine.
Sprinkle the crumble topping over the fruit mixture. Bake for 35 to 40 minutes or until bubbling and golden brown on top. Let cool slightly; serve with vanilla ice cream.
MAKE AHEAD: This can be made a day ahead and reheated in the oven (let come to room temperature before putting the dish in the oven – otherwise the dish could break!).
