Preheat oven to 350F.
In a large mixing bowl, whisk together almond flour, coconut flour, lupin flour, oat fiber, baking powder, baking soda, xanthan gum, and salt.
In a second large mixing bowl, combine the sour cream, melted butter, lemon zest, lemon juice, heavy cream, and monk fruit sweetener. Whisk well to combine.
Add the eggs and vanilla extract to the second mixing bowl and whisk again until small bubbles can be seen at the top of the mixture.
Combine wet and dry ingredients, using a spoon to mix well. Stir until a thick dough has formed.
Gently fold in blueberries to mixture.
Line a muffin tin with liners. Lightly grease each liner with oil, butter, or cooking spray.
Scoop muffin mixture evenly into the 12 muffin cups.
Bake for 20-22 minutes or until muffins have risen and the tops are a nice golden brown color. You'll know the muffins are ready when a toothpick comes out clean.
Remove from oven and allow to cool for 10-15 minutes. Remove from tin. Top warm muffins with a slice of butter and enjoy your lupin flour muffins!
