Preheat oven to 350°. Arrange muffin liners in the muffin tin and spray with vegetable spray. Set aside.
In a medium bowl combine the arrowroot flour, coconut flour, baking soda, sea salt and lemon zest. Whisk to combine.
In a large bowl, with a hand mixer beat together the honey and butter until smooth. Add the eggs and mix until smooth. Add the dry ingredients and mix until just blended. Fold in the blueberries and let the batter rest for 5 minutes.
Meanwhile, make the streusel. Combine the arrowroot flour, hazelnut flour, sugar, sea salt, cinnamon and butter in a small bowl and use your fingers to work it together to form the crumble.
Divide the batter among the muffin tins and top with the streusel. Bake for 20-25 minutes or until golden brown and set. Let the muffins rest for 5 minutes in the pan. While muffins are cooling, make the (optional) glaze. Stir together the confectioner's sugar and cream until smooth. Drizzle the glaze over the cooled muffins and serve.
For mini muffins bake 10-14 minutes. These muffins freeze well - but if you're going to glaze them -- wait until they've thawed from the freezer, then add the glaze.