Put the shells into a large saucepan and add all the other ingredients. Slowly bring to a simmer. Continue to simmer very gently over the lowest possible heat for about 15 minutes, to infuse the oil with the flavours from the shells and aromatics.
Remove the pan from the heat and set aside to infuse for 2-3 hours.
Strain the infused oil through a muslin-lined sieve and store in