Preheat the oven to 350℉.
Slice the nectarines (or peaches) in half, remove the pit, and cut into large wedges.
I don't find it necessary to remove the skin.
Place in a large bowl and drizzle with the lemon juice. Sprinkle the cornstarch over the fruit and gently mix.
Place the cut nectarines in a 8 or 9 inch baking dish. Dot with the blackberries, if using.
For the Streusel Topping
In a medium bowl, whisk together oats, flour, sugars, salt, and cinnamon. Pour in the melted butter and stir with a fork to form crumbles.
Sprinkle the topping over the nectarines.
Bake for 20-30 minutes until the topping is golden brown and crisp.
Allow the crisp to cool for at least 10 minutes.
Serve on it's own, or with a scoop of vegan vanilla ice cream.