Prep the peaches. If using fresh, peel if you like (the skins are fine to leave on). Slice into wedges. If using frozen, thaw and drain excess liquid. You’ll want about 6 cups of sliced peaches.

Season the fruit. In a bowl, toss peaches with 1/3 cup granulated sugar, 2 tablespoons brown sugar, 1–2 teaspoons lemon juice, 1 teaspoon lemon zest, 1 teaspoon vanilla, 1/2–1 teaspoon cinnamon, and 1–2 tablespoons cornstarch. Stir gently to coat.

Preheat and butter the dish. Heat oven to 375°F (190°C). Lightly butter a 9x13-inch baking dish or a large skillet. Spread the peaches evenly in the dish.

Make the topping. In a bowl, whisk 1 cup flour, 1/2 cup granulated sugar, 1/4 cup brown sugar, 1 1/2 teaspoons baking powder, and 1/2 teaspoon salt. Stir in 6 tablespoons melted butter and 2/3 cup milk until just combined. The batter will be thick but scoopable.

Top the peaches. Dollop spoonfuls of batter over the fruit, leaving a few gaps for steam. Don’t worry about perfection; rustic is the goal. If you like, sprinkle 1–2 teaspoons turbinado sugar on top for extra crunch.

Bake until golden and bubbly. Bake 35–45 minutes, until the topping is set and browned and the fruit is bubbling around the edges. If the top browns too fast, tent loosely with foil for the last 10 minutes.

Rest and serve. Let it sit 10–15 minutes so the juices thicken. Serve warm with vanilla ice cream or a dollop of whipped cream.