Prepare your campfire or coals: you want a bed of hot, glowing coals (not tall flames). For a 10-inch dutch oven, you'll need about 18 coals total — 9 underneath and 9 on top.
In a mixing bowl, combine blueberries, maple syrup, chia seeds, vanilla, and lemon zest (if using). Stir and set aside while you make the topping.
In another bowl, whisk together whole wheat flour, rolled oats, coconut sugar, baking powder, and salt. Cut in the cold butter using your fingers or a fork until the mixture looks like coarse crumbs.
Pour the buttermilk into the flour mixture and stir just until a shaggy dough forms. Do not overmix.
Spread the blueberry mixture evenly in the bottom of a 10-inch dutch oven. Drop spoonfuls of the dough over the top — it will spread as it bakes.
Cover the dutch oven with its lid. Place 9 hot coals underneath the pot and arrange 9 coals on top of the lid. Cook for 30-35 minutes, rotating the pot and lid a quarter turn every 10 minutes for even heat.
Check for doneness: the topping should be golden and firm, and the blueberries bubbling at the edges. If the top is browning too fast, remove a few top coals. If it's too pale, add 2-3 more.
Carefully remove the dutch oven from the coals using heavy gloves. Let it rest for 5 minutes before serving. Scoop into bowls and enjoy warm.
