Prepare hot coals in a campfire or chimney. You'll need 18 coals to start. They are ready when they turn white.
In a large bowl, mix the oats, baking powder, cinnamon, and salt together.
In a separate bowl or jug, whisk the milk, egg, and maple syrup together.
Pour the wet ingredients into the dry and mix until well combined. Stir in the blueberries.
Scoop the oat mixture into a parchment-lined 4-quart cast iron Dutch oven.
Cover with the lid and place 6 coals below the oven and 12 on the lid. Cook, replacing the coals as needed until the baked oatmeal is slightly puffed and set, 45 to 60 minutes.
Serve warm, topped with Greek yogurt and more maple syrup if desired.
Notes
Keep it level--Try to keep the Dutch oven as level as possible when cooking. If the oven is tilted, all the liquid will flow to one side, resulting in uneven cooking.
Using a different Dutch oven--If your Dutch oven is larger than 4 quarts, you will need more coals on the top and bottom and your cooking time will likely be 5 to 10 minutes shorter. When increasing the amount of coals, always maintain a ratio of 2 coals on top for every 1 below the Dutch oven.
Protect your oven--Since you're cooking in a Dutch oven outside, the cooking time can vary due to outside temperature and wind. If you're cooking on a very windy day, try to protect the Dutch oven with a barrier, such as a tarp.
Dietary--This recipe is suitable for gluten-free and vegetarian diets.