Roast and grind the hazelnuts in advance. Roast them for 15-20 mins at 180ºC/360ºF. Let them fully cool before removing the skin by rubbing them between your hands (or a clean dish towel). Then put into a food processor and pulse until fully ground down. Don't leave the processor running or you may release the oils and turn the hazelnuts into hazelnut butter.
Preheat the oven to 180°C /360°F.
Wash, peel and cut the apple into pieces. Grate if you don't have a food processor.
Put the apple. milk, agave syrup and sunflower oil in a mixer/food processor and mix/blend.
Add all the dry ingredients and mix.
If you are using blueberries, transfer the batter into a bowl, add the blueberries and mix them within the batter with a wooden spoon.
Grease each muffin cup with a bit of coconut oil.
Fill each muffin cup with batter.
Bake for about 25 minutes (when you insert a knife inside, it should come out clean).
Store in an airtight container at room temperature for 2-3 days, or freeze for up to 2 months.