Heat a non-stick frying pan over a medium heat and fry the chorizo for 3-4 minutes until it turns golden and releases its oil. Remove with a slotted spoon and set aside. Add the onion, garlic and rosemary to the pan, and cook for 3-4 minutes, until they begin to soften. Add the pearl barley and stir well to coat in the oil, before tipping in the chopped tomatoes and the chicken stock. Bring just to the boil, then turn down to a simmer and cook for approximately 35 minutes, stirring occasionally, until all the liquid has been absorbed and the pearl barley is almost tender. You may need to top up with a little extra water as you go. Add in the mussels, chorizo sausage and black olives and put the lid on the pot. Continue to cook for 5 minutes until the mussels are cooked and opened up. Serve immediately.