Choose only the good mussels and discard those with broken shells. Wash the mussels thoroughly until clean.
Bring 1 inch of water to a boil in a large covered pot. Add the mussels, cover the pot and let them steam for only 3 minutes to open the shells. Remove the top shell and loosen the mussels from the half of the shells that they are attached to before laying them in a baking dish.
Spoon about a teaspoonful of marinara sauce over each mussel in their shells.
Over medium heat melt the butter in a small saucepan and add the the garlic. Cook for just a minute to soften it.
Add the bread crumbs, chopped thyme and black pepper. Stir to combine well, then toss in the Parmesan cheese. Spoon the crumbs evenly over the marinara sauce covered mussels. Press the crumbs down gently and bake at 375 degrees F for 15 minutes.
Serve immediately while piping hot.
