Step 1: Preheat your oven to 375°F (190°C). Lightly butter a 9×13-inch baking dish.
Step 2: Prepare the peaches. If using fresh peaches, bring a pot of water to a boil and prepare an ice bath. Score a small ‘X’ at the bottom of each peach. Briefly dunk peaches into boiling water for 30-60 seconds, then immediately transfer to the ice bath. The skins should slip off easily. Peel, pit, and slice your peaches into 1/2-inch thick wedges. You’ll need about 6 cups. If using frozen, thaw and drain them well.
Step 3: In a large bowl, combine the sliced peaches with 1/4 cup granulated sugar, 1/4 cup light brown sugar (packed), 1 tablespoon all-purpose flour (or cornstarch for a gluten-free option), 1 teaspoon vanilla extract, 1/2 teaspoon ground cinnamon, a pinch of nutmeg, and 1 tablespoon fresh lemon juice. Toss gently until the peaches are well coated.
Step 4: Pour the peach mixture evenly into the prepared baking dish.
Step 5: In a separate medium bowl, whisk together 1 cup all-purpose flour, 1/2 cup old-fashioned rolled oats, 1/2 cup light brown sugar (packed), 1/4 cup granulated sugar, 1/2 teaspoon ground cinnamon, and 1/4 teaspoon salt.
Step 6: Add 1/2 cup (1 stick) unsalted butter, cold and cut into small cubes, to the dry ingredients. Using your fingertips, a pastry blender, or a fork, cut the butter into the flour mixture until coarse crumbs form. You want a texture resembling wet sand with some larger pea-sized clumps.
Step 7: Evenly sprinkle the crumble topping over the peach mixture in the baking dish. Do not press it down.
Step 8: Place the baking dish on a baking sheet (to catch any potential overflow) and bake in the preheated oven for 40-50 minutes, or until the topping is golden brown and the peach filling is bubbly and tender when pierced with a knife.
Step 9: Once baked, remove the crumble from the oven. Let it cool for at least 15-20 minutes before serving. This crucial cooling time allows the filling to set slightly. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream.