How to Make It
Preheat oven to 350°F. Gently toss together peaches, blackberries, granulated sugar, cornstarch, and lemon juice in a large bowl. Transfer fruit to a lightly greased 11- x 7-inch baking dish.
Toss together oats, flour, honey, kosher salt, cinnamon, cardamom, and nutmeg in a medium bowl; add cubed butter. Using your hands, combine until mixture is crumbly. Sprinkle topping evenly over fruit.
Bake in preheated oven until top is golden brown and fruit mixture is bubbling, 45 to 50 minutes. Remove from oven, and cool 15 minutes. If desired, sprinkle with lemon zest before serving.