1. Rinse the mussels well under cold water. Pick them over, pulling off any beards and discarding any mussels that are broken or gaping open.
2. In a large pot with a lid, heat the oil and butter over medium heat. Add the garlic and spring onions and cook until fragrant and soft but not coloured, 3 to 5 minutes.
3. Add the mussels, wine, and half of the parsley. Increase the heat to high, and cover the pan. After 2 minutes, remove the lid and toss the mussels well with a large spoon.
4. Cover the pot again and cook until the mussels have opened wide, another 3 to 4 minutes.
5. Add the remaining parsley, give the mussels a final toss, and divide the mussels and the broth among bowls. Serve with bread and lemon.