FOR THE TOPPING:
1 1/2 cups/190 grams all-purpose flour
1/2 cup/50 grams rolled oats
1/3 cup/75 grams light or dark brown sugar
1/3 cup/65 grams granulated sugar
1/4 teaspoon kosher salt
1 teaspoon ground cinnamon
1/2 teaspoon ground cardamom, ginger or allspice, or use lemon zest
1/2 cup/115 grams unsalted butter (1 stick), melted and cooled
FOR THE FILLING:
2 to 5 tablespoon light brown sugar (or granulated sugar), depending on the sweetness of the fruit
2 tablespoons cornstarch
8 cups mixed berries (fresh or frozen), or cubed peaches, plums, nectarines, apricots, or pitted sweet cherries (or a combination)
Ice cream or whipped cream, for serving
Heat oven to 350 degrees.
Make the topping: In a large bowl, whisk together flour, oats, sugars, salt and spices. Stir in butter. Using your hands, squish mixture until coarse crumbs form. Some should be about 1/2-inch in size, some smaller.
Spread topping in one layer onto a rimmed baking sheet. (You don’t have to grease it first.) Bake until crumbs are solid when you gently poke them, and are fragrant, about 15 minutes. They won’t change appearance very much. Transfer baking sheet to a wire rack to cool while you make the filling. (Crumbs can be baked up to 2 days ahead and stored in an airtight container at room temperature.)
Prepare the filling: In a large bowl, whisk together sugar and cornstarch until well combined. Add fruit and gently toss to coat with the sugar mixture. Pour filling into an ungreased 2-quart gratin dish or 10-inch cake pan, mounding the fruit in the center.
Spoon crumbs over filling and place the crumble dish on a rimmed baking sheet to catch any overflowing juices. (You can use the same baking sheet you cooked the crumbs on.) Bake until filling bubbles energetically around the edges, about 55 to 65 minutes. Let cool slightly. Serve warm or at room temperature with ice cream or whipped cream, if you like. Crumble can be made up to 8 hours ahead and kept at room temperature, or warmed up briefly in a 350-degree oven.