To shave the brussel sprouts, cut each one in half lengthwise and then lay them face down on a cutting board and slice into very thin shreds. Place in a bowl and set aside.
Cook the bacon in a skillet on medium heat until crisp, approx 5 minutes. Roughly chop into small pieces and then set aside.
Add the diced onion and garlic to the skillet with the bacon fat. Cook until the onions soften and then add in the shaved brussel sprouts, lemon juice and stock. Cook the brussel sprouts for approximately 7 minutes, stirring frequently, until they have softened and the water has evaporated.
Stir in the chopped bacon, hazelnuts, parsley and lemon zest. Serve warm.