Melt the butter over medium heat.
Add shallots, garlic, thyme and cook over medium low heat for 1-2 minutes until shallots are softened and fragrant.
Add the mustard and wine, stirring to combine.
Bring sauce to a boil, cover with a lid and then allow to simmer over the lowest heat for about 5 minutes.
Taste for salt & pepper and adjust as necessary.
Set aside the sauce for a few hours at room temperature; bring back to a boil when you are ready to use; or continue.
Scrub and de-beard mussels, if necessary.
Bring sauce to a boil, drop in the mussels and place a lid over for 2-4 minutes or until mussels are cooked (open).
Discard any mussels that did not open, serve immediately.