Preheat your oven to 350°F and line a muffin tin with silicone liners.
In a large bowl, whisk 0.5 cup of the Greek yogurt with the egg whites, 3 tsp of the lemon juice, and the vanilla extract.
Stir in the protein powder, almond flour, baking powder, sea salt, and lemon zest until a smooth batter forms.
Gently fold the blueberries into the batter using a spatula to keep them intact.
In a small separate bowl, finely chop the walnuts and mix them with the rolled oats and honey to create the crumble topping.
Divide the batter evenly into the muffin liners and sprinkle the walnut-oat crumble over the tops.
Bake for 20 to 22 minutes or until the muffins are golden brown and a toothpick inserted into the center comes out clean.
Whisk the remaining 0.1 cup of Greek yogurt and 1 tsp of lemon juice together, then drizzle over the muffins once they have cooled slightly.