Preheat your oven to 350°F (175°C). This ensures that the crisp cooks evenly and the topping becomes golden brown.

In a large mixing bowl, combine the sliced peaches, granulated sugar, lemon juice, cornstarch, and ground cinnamon. Toss gently until the peaches are evenly coated. The sugar will help draw out the juices from the peaches, creating a delicious filling.

Transfer the peach mixture into a 9-inch baking dish, spreading it out evenly.

In a separate mixing bowl, combine the rolled oats, all-purpose flour, brown sugar, and salt. Mix well to ensure the dry ingredients are evenly distributed.

Pour in the melted butter and vanilla extract. Use a spatula or your hands to mix until the mixture resembles coarse crumbs. This texture is crucial for achieving a crunchy topping.

Sprinkle the oat mixture evenly over the peach filling in the baking dish.

Place the baking dish in the preheated oven and bake for 30-35 minutes, or until the topping is golden brown and the peach filling is bubbling around the edges. The bubbling indicates that the filling is cooked through.

Once done, remove from the oven and let it cool for about 10 minutes before serving. This cooling time allows the juices to thicken slightly, making it easier to serve.