Preheat the oven to 350°F.
In a large bowl, whisk together the eggs, sweetener, vanilla extract, and Greek yogurt.
Grind the old-fashioned oats into a fine flour using a blender or NutriBullet.
Sift the ground oats to remove large clumps, then add them to the wet ingredients.
Stir in the baking powder.
Add the almond milk and mix until you have a thick, smooth batter.
Allow the batter to rest for 30 minutes to 1 hour, or refrigerate overnight if preparing ahead.
Lightly butter a piece of parchment paper and place it in a 9-by-9-inch baking dish.
Pour the batter into the dish.
Sprinkle the blueberries and chopped walnuts over the top.
Bake for approximately 45–50 minutes, or until the top is lightly browned and the cake is set.
Run a knife around the edges to loosen the cake from the pan, then carefully remove it.
Allow it to cool before slicing, or enjoy it warm.
Dust with powdered sugar if desired.
Cut into 9 servings.