In a large pot, combine water, 0.25 liter white wine, 1 bunch soup greens, 3 sliced onions, 1 tablespoon butter, salt, 5 bay leaves, and 15 grains pepper.
Bring to a boil over medium-high heat and cook for 15 minutes.
Add the mussels to the pot, adding more water if needed to completely cover them.
Return to a boil and cook for 6 minutes.
Discard any mussels that did not open during cooking.
Transfer the mussels to a preheated terrine and serve.