1. Place the mussels in a large bowl of cold water. Using a small sharp knife, scrape off barnacles and remove any brown wispy bits that hang out of the shells. Discard any mussels with broken shells or if they don't close when tapped. Drain the mussels in a colander.
2. In a large saucepan, heat the olive oil until it shimmers. Add the onion and garlic, season with pepper and cook until the onion is softened, about 5 minutes. Add the cream and cook, stirring often for 2 minutes. Add the Guinness and let simmer for 5 minutes until slightly reduced.
3. Add the mussels, cover tightly and turn the heat down. Steam for 5-6 minutes, shaking the pan halfway through until the shells have opened. Discard any unopened shells.
4. To serve, divide the mussels into bowls with the sauce, sprinkle with basil and serve with warm crusty bread.