Preheat oven to 375°F. Mix the swerve and xanthan gum in a cup, then set aside. Add the berries, lemon juice, and zest to a large bowl. Sprinkle the sweetener and xanthan gum over the top and toss until well coated. Pour the mixture into a buttered glass or ceramic baking dish. In a separate bowl, add all the crumble topping ingredients, except for the brown sweetener and butter, then whisk. Mix in the brown sweetener and melted butter. Sprinkle the crumble over the top of the berries. Bake for 30 minutes, then increase the temperature to 425°F and bake for an additional 5 minutes or until the crumble is crisp. Allow to cool slightly before serving with a scoop of Keto Vanilla Ice Cream or whipped cream on top.