Bring a large pot of water to a boil and prepare a bowl of ice water. Score an X on the bottom of each peach, blanch for 30 seconds, then transfer to the ice bath. Peel the skins off, slice into 1/2-inch thick slices, and discard pits.
Preheat your oven to 350°F (175°C). Lightly butter the inside of a 9x13 inch baking dish.
In a large bowl, combine sliced peaches with granulated sugar, cornstarch, lemon juice, vanilla, cinnamon, nutmeg, and salt. Toss gently to coat, then let sit for 5 minutes. Pour the mixture into the prepared baking dish.
In a medium bowl, whisk together flour, oats, brown sugar, cinnamon, and salt. Cut in the cold butter using a pastry cutter until the mixture resembles coarse, pea-sized crumbs. Stir in the pecans if using.
Evenly sprinkle the crumble topping over the peach filling, covering all surfaces including corners and edges. Do not press down; keep it light and loose.
Bake on the middle rack for 40 to 45 minutes until the topping is deep golden brown and the filling is bubbling at the edges. Tent with foil if browning too quickly.
Let the crisp cool on a wire rack for at least 15 minutes before serving to allow the filling to thicken. Serve warm, with ice cream or whipped cream if desired.