Preheat oven to 350 and grease a standard muffin pan.
In a small bowl mix together the flax and water and let sit for 5 minutes.
In a high powdered blender or food processor process hazelnuts to a powder. (I like to measure out the amount for the streusel here too and set it aside.) Don't over blend or you'll make hazelnut butter.
Add all other ingredients for the muffins (except oats but including the flax seed mix) to the blender or food processor and blend just until smooth. Stir in oats. Let sit while you make the streusel topping.
For the hazelnut streusel
In a small bowl combine all of the streusel ingredients using a fork. It should have some powder and some crumbs.
Use a cookie scoop or spoon to divide the batter out into the muffin pan. Top with streusel.
Bake at 350 for 27 minutes. Remove from oven and let cool slightly (5 min) before removing from pan or serving. Store in a air tight container at room temp for 3 day or in the fridge for a week. These muffins also freeze well.
You can make hazelnut meal just by pulsing roasted hazelnuts in a food processor or blender a few times until they are a course flour-like powder. If you can find pre-roasted, unsalted hazelnuts those are the best to use!