1. In a large stockpot, heat the butter over medium-high heat and add the garlic and onions. Sauté 4 to 5 minutes, until onions are glassy.
2. Pour in the water, wine, and vermouth and bring to a boil. Add the mussels, shake the pot to distribute evenly, and cover. Cook until the shells open, 5 to 6 minutes. Remove and discard any unopened shells.
3. To serve, place mussels in a serving bowl filled with the cooking broth and set on a table with crusty bread for dipping.
Note: Use the mussel shell like a tweezer to remove the mollusk.