Preheat oven to 350 degrees Fahrenheit. In a 13x9 baking dish, add peaches.
In a large oval baking dish, add peaches. Toss with granulated sugar, brown sugar, cornstarch, lemon juice, lemon zest, vanilla extract, and cinnamon.
4 cups (~6) firm peaches, 1/3 cup granulated sugar, 1/4 cup light brown sugar, 2 teaspoons vanilla extract, 2 Tablespoons cornstarch, 2 Tablespoons lemon juice, zest of half a lemon, 1/2 teaspoon cinnamon
In a separate bowl, whisk the topping ingredients (oats, flour, brown sugar, butter). Pour evenly over peaches.
1 cup oats, 1 cup flour, 3/4 cup brown sugar, 10 Tablespoons butter
Bake for 30-35 minutes or until the edges are bubbling and the top is golden brown and bubbly.
Let cool 5-10 minutes. Serve with ice cream, if desired.