In a wide skillet with a lid, melt butter
over medium-high heat. Add onion
, and stir until very soft and beginning to turn gold, about 5 minutes; do not let them brown
. Add thyme
and bay leaf and stir. Add wine
and 1 cup water and bring to a boil over high heat. Add mussels
, cover, and cook
until open, about 4 minutes.