Preheat the oven. Set it to 375°F (190°C). Lightly butter a 9-inch square baking dish or similar 2-quart casserole.

Prep the fruit. Wash, dry, and slice larger fruit into bite-size pieces. Halve or quarter smaller stone fruits and pit cherries if using. You want roughly 5–6 cups total.

Mix the filling. In a large bowl, combine fruit with sugar, lemon juice, cornstarch, vanilla (if using), and a pinch of salt. Toss until the fruit is glossy and the starch is evenly distributed. Taste and adjust sugar if your fruit is very tart.

Make the crumble topping. In another bowl, stir together flour, oats, brown sugar, cinnamon, and salt. Add the cold butter cubes. Use your fingertips or a pastry cutter to work the butter into the dry ingredients until you have pea-size clumps. Mix in the nuts if using. The mixture should feel slightly moist and crumbly.

Assemble. Spread the fruit in an even layer in the baking dish. Sprinkle the crumble topping evenly over the fruit, leaving some small gaps to let steam escape.

Bake. Place the dish on a rimmed baking sheet to catch any drips. Bake for 35–45 minutes, until the topping is golden and the fruit is bubbling around the edges. If the top browns too fast, tent loosely with foil.

Rest, then serve. Let the crumble cool for at least 15–20 minutes so the juices thicken. Serve warm with ice cream, whipped cream, or yogurt.