To Make Filling:

1. Slice off tops and bottoms of peaches; this will make them easier to peel. Using a vegetable peeler, carefully remove skin from each peach. (Nectarines do not need to be peeled; their skin will cook nicely into the crisp.)
2. Place peaches cut-side down on a cutting board. Use a paring knife to cut each peach in half. Remove pits. With halves facedown, cut peaches into bite-size pieces. Prepare nectarines same way.
3. In a medium bowl, combine the 1 tablespoon of flour, the 2 tablespoons of sugar, the peaches, and nectarines. Add blueberries (if using) and stir to mix with a wooden spoon. Scrape fruit mixture into a 9-inch glass or ceramic baking dish.

To Make Topping:

1. Position an oven rack in middle of oven. Preheat oven to 375°. In a clean medium-size bowl, whisk together the [q:2/3] cups of flour, the oats, brown sugar, cinnamon, and salt until cinnamon has completely mixed in.
2. Cut butter into small pieces ([q:1/4]-inch square). Add butter to flour mixture. With clean hands, massage mixture gently until it just comes together like a loose dough. Sprinkle topping over filling in a thin layer.
3. Bake on middle rack until filling is bubbling at surface and top is golden brown, 35 to 40 minutes. Remove from oven and allow crisp to cool for 5 minutes.
4. Spoon crisp into bowls while it’s still warm. Serve with a scoop of vanilla ice cream, if desired.