In a large bowl, toss the peaches with the flour, sugar, lemon juice, and vanilla. Spread evenly in the prepared dish.

Make the topping: In a medium bowl, whisk together the oats, flour, brown sugar, cinnamon, and salt. Use a fork, a pastry cutter, or your hands to work in the butter until the mixture is crumbly. Sprinkle evenly over the peaches.

Bake in a preheated oven at 350°F (175°C) for 30 minutes or until the topping is golden brown.

Let stand for 10 minutes before serving with vanilla ice cream.