Place all the dressing ingredients in a jug and mix thoroughly.
Place in a glass jar or suitable container with a lid and keep in the fridge until you are ready to use.
Place the onion, garlic, Brussels sprouts, celery and oil in a frying pan and turn the heat to medium.
After 10 minutes, add the frozen peas and stir through.
Allow to cook for a further 2 minutes.
Serve warm with a generous drizzle of the dressing, a sprinkling of hazelnuts and season to your taste.
You can eat this cold too so save some for lunch tomorrow!