Preheat the oven to 350°F (175°C). Lightly grease an 8x8-inch baking dish or a similar 2-quart dish.
Prepare the berries: In a large bowl, combine berries, sweetener, vanilla, lemon juice, lemon zest, and salt.
If using chia seeds, sprinkle them in and toss. If using xanthan gum, sprinkle it lightly over the berries and fold gently to avoid clumps.
Transfer the berry mixture to the baking dish and spread it into an even layer.
Make the crumble: In a separate bowl, mix almond flour, chopped nuts, sweetener, cinnamon, and salt. Add the cold butter and vanilla.
Use your fingers or a pastry cutter to work the butter into the dry ingredients until coarse crumbs form.
Top the berries: Sprinkle the crumble evenly over the berry layer, leaving some small gaps for steam to escape.
Bake for 28–35 minutes, until the topping is golden and the berries are bubbling around the edges. If the top browns too quickly, tent loosely with foil for the last 10 minutes.
Cool for 10–15 minutes before serving to let the juices thicken slightly. Serve warm with a dollop of whipped cream if you like.