Scrub mussels with brush. Discard any that are broken. If they are open, tap lightly on the counter. If the mussel is good, it will close. Discard any that are questionable.
Peel ginger and cut two pieces, each one inch long
Peel and coarsely chop shallots
Melt butter on high heat, and add olive oil
Toss ginger and shallots into pan. Cook on high heat until nicely browned (about three minutes) Leave in pan for flavor.
Add the full bottle of Guinness
Add mussels and the water and place lid over the top of pan. Stir when needed to expose mussels to heat. Cook on high heat for about five minutes.
Mix mustard into heavy cream and pour the mixture over the top of the mussels. Mix to coat and cook another minute.