Thaw the frozen peaches in a colander set over a bowl at room temperature for about an hour or overnight in the refrigerator. Press gently to release excess liquid, pat dry with paper towels if needed, and discard the collected juice.
Preheat oven to 350°F (175°C) and lightly grease a 9x13 inch baking dish with butter or nonstick spray.
In a large bowl, combine the drained peaches, granulated sugar, cornstarch, lemon juice, cinnamon, nutmeg, salt, and vanilla. Toss gently to coat evenly, let sit for 5 minutes, then pour into the prepared baking dish.
In a medium bowl, whisk together oats, flour, brown sugar, cinnamon, and salt. Cut in the cold butter with a pastry cutter until the mixture resembles coarse crumbs, then stir in the chopped pecans.
Sprinkle the topping evenly over the peach filling, covering the fruit completely. Do not press down the topping.
Bake for 35 to 40 minutes until the topping is deep golden and the filling is bubbling around the edges. Tent with foil if the topping browns too quickly.
Cool on a wire rack for at least 15 minutes before serving to allow the filling to set. Serve warm with vanilla ice cream or whipped cream.