In a large, heavy saucepan, bring 1 cup water to a boil. Wash mussel shells and add to boiling water. Cover and cook until mussels open. Discard any mussels that have not opened. Drain the mussels and let cool enough to handle. Pull them out of the shells and snip off the beard with a pair of scissors. Set mussels aside and keep warm. Discard half of the shells and reserve the others.
Clean the spinach, trim off any large stems and plunge into a fresh pot of boiling water 3 to 4 minutes. Drain well. When cool enough to handle, chop spinach coarsely. Keep warm. In a small skillet, combine shallots, wine and lemon juice. Reduce over low heat until liquid is nearly gone (watch carefully as the reduction will scorch easily at this point). Remove pan from heat and add one or two pieces of butter. Stir steadily with a whisk until the butter is no longer in a solid form, yet not liquid either. It should resemble the consistency of a thin mayonnaise. Incorporate the rest of the butter, one or two pieces at a time, returning the pan to a low heat as necessary, retaining the same consistency. If butter separates, the heat is too high; remove it from the heat and whisk rapidly to re-emulsify. Set sauce aside and cover to keep warm. To serve, place about 1 teaspoon of the warm spinach in each reserved half -shell and top with mussel. Pour 2 teaspoons warm sauce over each. If desired, garnish with slivers of red pepper.