To vary the filling here, use 4 pounds of berries (strawberries, blackberries and raspberries); or 4 pounds of plums, cut into 1-inch cubes; or 6 pints of blueberries plus 2 tablespoons of fresh lemon juice.
4 pounds stone fruit (peaches, nectarines and apricots) cut into large wedges
3/4 cup granulated sugar
2 tablespoons cornstarch
1 1/3 cups all-purpose flour
2/3 cup light brown sugar
1 teaspoon baking soda
1 teaspoon kosher salt
1 stick plus 2 tablespoons cold unsalted butter, cubed
1 cup thick-cut rolled oats
In a bowl, toss the fruit with the granulated sugar and cornstarch and let stand for 10 minutes.
Preheat the oven to 375°. In a food processor, pulse the flour with the brown sugar, baking soda and salt. Add the butter and pulse until crumbly. Add the oats and pulse until incorporated. Transfer the topping to a bowl and press it into clumps.
Spread the filling in a 9-by-13-inch baking dish. Scatter the topping over the filling. Bake in the center of the oven for 1 hour, until the fruit is bubbling and the topping is golden and crisp. Let cool for 1 hour before serving.