Preheat the oven to 350 degrees. Coat two mini baking dishes or ramekins with cooking spray.
In a large bowl, combine the blackberries, sliced peaches, and sliced plums and add the sugar, lemon zest, vanilla, and cornstarch, then gently stir to mix thoroughly.
Pour into the two mini baking dishes or ramekins.
Make the topping by combining the oatmeal, brown sugar, cinnamon, and butter; mix well with your fingers until crumbly.
Spoon evenly on top of the fruit.
Place on a baking sheet covered with parchment paper (for easier clean up) and place into the oven to bake for 30 minutes.
Remove from the oven and serve with whipped cream or ice cream, if desired. Enjoy.