In a large skillet, heat butter, garlic, lemon juice and celery until soft. Mix in spinach, red wine vinegar, Old Bay seasoning and white pepper. Cook over low heat for 15 minutes. Add the heavy cream and mix well. Remove the empty half of the mussel shell and discard, place the other half shell (with the meat) on a sheet pan and top each mussel with about 1 teaspoon of sauce. Cook in a preheated 350 degree oven for an additional 4-5 minutes. Serve immediately. Garnish with lemon wedges and sprig of parsley. Serves 4-6.