Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

In a large bowl, gently toss together the sliced strawberries and peaches. Add the granulated sugar, cornstarch, and lemon juice. Stir until the fruit is evenly coated. Pour the fruit mixture into the prepared baking dish and spread evenly.

In a separate medium bowl, combine the flour, rolled oats, brown sugar, cinnamon, and salt. Whisk to combine.

Add the cold, cubed butter to the dry ingredients. Using a pastry blender, your fingertips, or a fork, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces.

Evenly sprinkle the crisp topping over the fruit filling in the baking dish.

Bake for 35-45 minutes, or until the topping is golden brown and the fruit filling is bubbly and thickened. If the topping starts to brown too quickly, you can loosely tent it with foil for the last 10-15 minutes of baking.

Let the crisp cool for at least 10-15 minutes before serving to allow the filling to set. Serve warm, optionally with a scoop of vanilla ice cream or whipped cream.