Preheat the oven: Set to 350°F (175°C). Lightly grease an 8x8-inch baking dish or similar.

Prepare the berries: In a bowl, combine blueberries, half of the sweetener (start with 3 tablespoons and taste), lemon juice, and vanilla extract. Add lemon zest and a tiny pinch of salt. If you prefer a thicker filling, sprinkle in xanthan gum or chia seeds and toss well.

Mix the crumble topping: In a separate bowl, stir together almond flour, coconut flour, remaining sweetener, cinnamon, and a pinch of salt. Add chopped nuts.

Cut in the butter: Add cold butter cubes to the dry mixture. Use your fingers or a pastry cutter to work it in until you have coarse, sandy crumbs with some pea-sized bits. If using coconut oil, chill the mixture for 10 minutes to firm it up.

Assemble: Pour the blueberry mixture into the prepared dish. Scatter the crumble evenly over the top without packing it down too tightly.

Bake: Place on the center rack and bake for 30–40 minutes, until the topping is golden and the berries are bubbling at the edges. If using frozen berries, add 5 minutes.

Rest: Let the crumble sit for at least 15 minutes to set. This helps the juices thicken and makes serving easier.

Serve: Spoon into bowls. Add a dollop of unsweetened whipped cream or a small scoop of keto ice cream if you like.