Preheat the oven to 180'C.
Peel the potatoes and cut into cubes. Cut the leeks into quarters lengthways and then chop. De-beard the mussels and check that they are alive, discarding any that are not. Pick the rosemary leaves and then chop.
Melt the butter in a saucepan and then sweat the leeks until soft. Season with salt and then add the potatoes, saffron and garlic. Add the whole sprigs of thyme to the pan and cook for 2-3 minutes before adding the white wine. Reduce the white wine by half and then add enough fish stock to cover the vegetables. Lower the heat and simmer until the potatoes are cooked. Add the mussels to the pan, cover with a lid and cook until all of the mussels are open.
For the crostini: place the bread slices on to a baking sheet and drizzle with olive oil. Place in the oven and cook for 10 minutes or until crisp and golden brown. Remove from the oven and then spread the goat's cheese on top. Sprinkle with rosemary cheese and return to the oven to melt the cheese.
Serve the crostini on top of the soup.
«For a vegetarian alternative, remove the mussels from this recipe and use vegetable stock.»
Leek, potato and mussel soup scented with saffron and served with a goat's cheese crostini.