To blanch the peaches, bring a large pot of water to a boil; place the peaches in the boiling water for one minutes, or just until the skins peel off easily.
Transfer the peaches to ice cold water.
Peel the peaches; remove the pits and cut into ½-inch thick slices.
Transfer sliced peaches to a bowl and toss with lemon juice.
Sprinkle with cornstarch and sugar and toss to combine.
Arrange peaches on the bottom of previously prepared baking dish and set aside.
To Make the Topping and Bake:
In a food processor fitted with a steel blade, combine flour, brown sugar, granulated sugar, cinnamon, salt, and nutmeg.
Pulse until everything is combined.
Add in the diced butter and pulse until the mixture resembles peas-sized crumbs.
Transfer to a bowl and, using your hands, mix in the rolled oats and pecans, if using.
Sprinkle the flour mixture evenly over the peaches in one layer.
Bake for 45 minutes, or until the top is a golden brown and the fruit is bubbly.