Heat oil in a large pot with high sides over high heat. Add the mussels to the pan, cover it and cook until the mussels open. Once ready drain, collect the cooking water, shell the mollusks and return them to the liquid. In a pan, brown the spring onions with butter and oil and then add the tomato paste.
Add the mollusks’ water (not the mussels) and bring to a boil. At this point add the basil sprig and the cream. Cook for about 5 minutes, filter and if needed add more cream to thicken. Serve in small soup bowls, with 3/4 shelled mussels on top, a drizzle of oil and a pinch grated bottarga. Finish with a basil leaf.