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Wash the mussels and pull off any beards that may still be attached to the shell. A pair of needle-nose pliers works well for this. Discard any mussels with cracked or broken shells and those that do not remain closed after a minute when you pinch them together. Also discard any mussels that do not open during the cooking time.
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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.
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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.
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Spoon about a teaspoonful of marinara sauce over each mussel in their shells.
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Over medium heat melt the butter in a small saucepan and add the the garlic. Cook for just a minute to soften it.
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Add the bread crumbs, chopped thyme and black pepper. Stir to combine well, then toss in the Parmesan cheese.
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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.