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

Mix the filling: In a large bowl, combine blueberries, lemon zest, lemon juice, 1/4 cup keto sweetener, vanilla extract, and chia seeds. If using very juicy berries, sprinkle in coconut flour to help thicken. Gently fold to coat.

Make the crumble topping: In another bowl, stir together almond flour, remaining 1/4 cup sweetener, chopped nuts, shredded coconut (if using), cinnamon, nutmeg, and salt.

Cut in the butter: Add the cold butter cubes to the dry topping mixture. Use a pastry cutter or your fingertips to blend until the mixture forms coarse, pea-sized crumbs.

Assemble: Pour the blueberry mixture into the prepared dish and spread evenly. Sprinkle the crumble topping evenly over the berries.

Bake: Place on the center rack and bake for 30–40 minutes, until the topping is golden and the blueberry juices are bubbling around the edges.

Rest: Let the crumble cool for at least 10–15 minutes before serving. This helps the juices set and makes scooping easier.

Serve: Enjoy warm, plain or with a dollop of unsweetened whipped cream or a scoop of sugar-free vanilla ice cream.