Preheat and prep: Heat your oven to 350°F (175°C). Lightly butter a 9-inch square baking dish or a similar 2-quart baking dish.

Prep the peaches: If using fresh peaches, peel them (a quick blanch helps) and slice into 1/2-inch wedges. If using frozen, thaw and drain excess liquid so the filling doesn’t get watery.

Mix the filling: In a large bowl, combine sliced peaches, lemon juice, granulated sugar, 1 teaspoon cinnamon, vanilla, and cornstarch. Toss until the peaches are evenly coated and glossy. Pour into the prepared baking dish and spread evenly.

Make the crumble topping: In another bowl, stir together flour, oats, brown sugar, remaining 1/2 teaspoon cinnamon, and salt. Add the cold butter cubes. Use your fingers or a pastry cutter to work the butter in until the mixture looks like coarse crumbs with some pea-sized bits.

Add crunch (optional): Fold in a small handful of chopped pecans or almonds for extra texture and flavor.

Top the peaches: Sprinkle the crumble mixture evenly over the peach filling. Don’t press it down—loose crumbs bake up crispier.

Bake: Place the dish on a baking sheet to catch any drips. Bake for 40–45 minutes, until the topping is golden brown and the filling is bubbling around the edges.

Rest before serving: Let the crumble sit for 10–15 minutes. This helps the juices thicken so you get spoonable, jammy peaches instead of a runny mess.

Serve: Spoon into bowls and top with a scoop of vanilla ice cream, a dollop of whipped cream, or a spoonful of tangy yogurt.