Put mussels, pilsner and cream in a large pot, cover with a lid, bring to a boil and steam for 2 to 3 minutes, or until mussels have opened.
Remove from heat and drain, reserving cooking liquid in a bowl and discarding any mussels whose shells remain closed.
Strain cooking liquid through a fine sieve, place in a pan and boil for 5 minutes or until thickened. Reserve for sauce.
Remove mussels from shells, separate top and bottom halves of shells, discarding top halves. Reserve remaining shells and mussels separately.
Melt butter over high heat in a large skillet. Sauté shallots, smoked bacon and mushrooms for 3 minutes, or until bacon is cooked and vegetables are soft.
Combine vegetable mixture with reserved mussels, reduced cooking liquid and chopped parsley.
Spoon mussels and mixture into reserved mussel shells. Place in a baking dish, top with grated cheese (at this point you can refrigerate for up to 24 hours until ready to serve) and bake for 5 minutes or until cheese is melted and mussels are warmed through.