Preheat your oven to 350°F (175°C). Lightly grease a 9-inch baking dish with butter or cooking spray. This helps prevent sticking and makes serving much easier after baking.

Wash, peel, and slice the peaches into even pieces. Place them into a large mixing bowl. Even slices cook evenly and create a soft, juicy filling.

Add the sugar, brown sugar, lemon juice, vanilla extract, cinnamon, salt, and cornstarch. Stir gently until every peach slice is coated with the mixture. This creates a thick, flavorful filling.

Pour the peach mixture into the prepared baking dish. Spread the peaches into an even layer. This helps every serving have plenty of fruit.

In another bowl, combine the flour, oats, brown sugar, cinnamon, and salt. Pour in the melted butter. Mix gently with a fork until small crumbs form.

Sprinkle the topping evenly over the peaches. Cover the fruit completely without pressing the topping down. Keeping it loose creates a crisp texture.

Place the baking dish into the oven. Bake for about 40 minutes. The topping should become golden brown. The peach filling should bubble around the edges.

Remove the crisp from the oven. Let it cool for about 15 minutes before serving. Enjoy it warm with vanilla ice cream or whipped cream for the best dessert.