Heat 1 tablespoon of the olive oil in a large pot, and when nice and hot add the mussels and fish stock. The stock should steam up immediately, so cover the pan and give it a shake and let the mussels steam for 3 – 4 minutes until they open. Drain the mussels, reserving the stock and mussel liquor. Measure the resulting stock, and make it up to half a litre (500ml) with water or extra stock and set aside. If desired, you can separate the mussel shells, leaving the half-shell with meat attached for serving.