(The topping can be made ahead of time and kept covered in the refrigerator for a few days or frozen for up to 3 months)
Preheat the oven to 350 degrees. In a large mixing bowl, combine the peaches, nectarines, frozen blueberries, lemon juice, sugar (if using), and flour. Fold the fruit over a few times to combine. Pour the fruit into a baking dish that is roughly 9″x12”.
In another large mixing bowl, combine all the ingredients for the topping. Using your hands, squeeze the butter and other ingredients together until small clumps are created. Pour the topping evenly over the fruit until it is covered.
Place the baking dish on a cookie sheet (this helps prevent fruit splatter and spills in the oven) and bake for 40-45 minutes or until the crisp is bubbling and the topping is golden brown.
Remove the crisp from the oven and let cool for a few minutes. It is delicious served on its own or with vanilla ice cream or homemade whipped cream.