Preheat oven to 375 F. Lightly grease a 10 or 12 inch iron skillet and set aside.
Thinly slice each peach (about 1/2 inch thick) and place the peach slices in a large mixing bowl. Add the raspberries, lemon juice, 3 Tablespoons of flour, granulated sugar and 1/2 teaspoon cinnamon. Gently stir about 3-4 times or until the flour is absorbed. Spoon the fruit mixture into the greased skillet.
In a medium mixing bowl, add the rolled oats, brown sugar, 1/3 cup flour, 1 teaspoon cinnamon and the melted butter. Toss to coat evenly. Sprinkle the oatmeal mixture on top of the fruit, covering the fruit entirely.
Bake for 25 to 28 minutes or until the fruit is bubbly and the oatmeal crisp is golden brown. Remove from oven.
Allow to cool for at least 10 minutes before serving.
Best served with a scoop of vanilla ice cream or whipped cream. Enjoy!