Preheat oven to 350℉. Lightly grease a 9x9 baking pan.
Peel, pit, and slice 3 pounds fresh peaches and place into a large mixing bowl. Toss with 1/4 cup lemon juice, 1/4 cup granulated sugar, 2 tablespoons cornstarch, 1/2 teaspoon ground cinnamon, and 1/2 teaspoon vanilla extract. Scoop into prepared baking pan with all the juices.
In a medium-sized bowl or food processor, combine 1/2 cup all-purpose flour, 1/2 cup brown sugar, and 1/2 teaspoon ground cinnamon Use a pastry cutter or food processor to cut in 1/2 cup salted butter (cold), until the mixture resembles cornmeal.
Stir in 1 cup rolled oats. Sprinkle mixture over the top of the peaches in an even layer.
Bake in the preheated 350 degree oven, uncovered, for 30 minutes until crumble is toasted and lightly browned.