Heat the olive oil in a large frying pan or paella pan and sauté the onions, pepper, tomatoes, mushrooms and garlic until golden and tender. Remove from the pan with a slotted spoon and set aside.
Add the prawns and sauté over a high heat until just pink. Add the vegetables back into the pan and sauté for two minutes.
Add the saffron to the fish stock then pour into the pan and bring to the boil. Add the spaghetti and fish and stir. Then add the mussels and cook for around 15 minutes at a fast boil until the pasta is al dente.
Season carefully before serving with the alioli on the side.