Preheat grill to medium high. Cut country loaf into slices. Place on grill to toast one side.
Meanwhile, cover bottom of a large straight sided and heat proof skillet with foil to protect from soot. Place pan on the grill and add all ingredients, except butter, parsley, bacon and black pepper, stirring to mix. Remove bread slices to a cookie sheet, toasted side up and grate some gruyere cheese over the slices. Return bread slices to the grill to toast the other side and melt the cheese.
While bread slices are toasting, tint skillet loosely with foil and cover the grill, until bread slices are toasted (about 1 minute depending on grill heat. Be sure to check so bread doesn't burn).
Open grill and remove bread slices to cookie sheet and set aside. Close grill and allow muscles to continue cooking about another 3 minutes. Remove the foil and if some have not opened, give the pot a stir and cook another 1-2 minutes.
Remove skillet from grill and remove and discard any muscles that have not opened. Scoop remaining mussels into a large serving bowl. Return skillet with liquid to the grill, and let simmer 1-2 minutes to combine flavors. Remove from heat, swirl in butter, and pour liquid over muscles in bowl.
Sprinkle with black pepper, crumbled bacon (if using) and parsley. Serve with toasted country bread.
A butter enrichment should always be added at the end of a recipe, after the liquid has cooled.