Start by making the crumble topping. In a large bowl, combine flour, brown sugar, sugar, cinnamon, and kosher salt. Whisk together, then cut cold butter into chunks. Add into the bowl and pinch the butter into the flour mixture (or use a pastry cutter) until pea sized morsels form (pro tip: it's well mixed when you can squeeze it in your hand and it sticks together). Place into the freezer to chill.
Cut peaches in half, remove the pit, then slice into about 1/4 inch slices.
In a separate large bowl, combine sliced peaches, brown sugar, cornstarch, cinnamon, ginger, and nutmeg. Toss together then add in the lemon juice, vanilla extract, and melted butter. Mix together to ensure all of the peaches are fully coated in the spices and cornstarch. Let the peaches sit for 10 minutes.
Add the peaches into a 10-inch cast iron skillet or 9x13 baking dish. Remove the crumb topping from the freezer. Sprinkle on top of the peaches (for larger clumps, squeeze together and break into big pieces).
Bake for 50-55 minutes in a cast iron skillet (or 35-40 mins in a 9x13 baking dish), until the peaches are bubbly and the top of the crumble is a nice golden brown. Let the peach crumble sit for 15-20 minutes prior to serving.
Notes
Get the best fresh peaches you can find! Go for ripe but firm peaches. They should give slightly when pressed but not feel mushy.
Peach crumble is best served fresh! It becomes soggy over time so keep this in mind when serving/preparing.
Use cold butter and freeze the topping before adding on top of the peach mixture. This ensures that the crumble doesn't bake into the peaches and that it stays nice and crisp on top.
Peel or don’t peel? I personally prefer to keep the peach skin on as it adds flavor and texture! However,If you like a super-smooth filling, add the peaches to boiling water for 30 seconds, then remove and place into a big bowl of ice water. Let them sit for 5 minutes, then peel the skins off.
Bake until bubbly. When the peach crumble is done baking, the filling should be visibly bubbling in the center and the topping deep golden brown. This ensures the fruit has thickened and the topping stays crisp.
Let it rest! Give it at least 20 minutes before serving so the juices set and slices get nice and syrupy and gooey!