Pre-heat your oven to 350 degrees F.
Grease a 9x13-inch casserole dish.
In a large bowl, toss peaches with cinnamon, vanilla, ½ cup granulated sugar and 3 tablespoons flour. Pour the peaches into the greased casserole dish.
In a separate large bowl, combine ½ cup brown sugar with ⅔ cup all-purpose flour, ⅔ cup old-fashioned oats, and 3/4 teaspoon ground cinnamon. Cut in the cold butter with two forks until coarse crumbs form. You can also use a food processor for this step.
Pour the crumbly topping on top of the peaches.
Bake in the oven, uncovered for 45-50 minutes, until the peach filling is bubbling around the edges and the topping is golden brown. If the topping is browning too quickly, tent with foil.
Rest: allow the peach crisp to rest for 10-15 minutes before serving. It is best served warm with a scoop of vanilla ice cream on top.