Heat oven to 180C/160C fan/gas 4. Lightly grease a silicone muffin tray with oil spray, or line with paper cases. Put the flour and hazelnuts in a food processor, and whizz until the nuts are finely ground. Tip into a bowl and stir in the bicarbonate of soda. Whisk together the buttermilk, milk, eggs, agave and oil.
Mix the wet ingredients into the dry until smooth, then stir in the blueberries. Divide the mixture between the muffin holes and bake for 20 mins until a skewer poked in comes out clean. Leave to cool for 10 mins, before turning out of the tray. If freezing, defrost at room temperature, then warm through in the oven.