Bring a large pot of water to a rolling boil. Salt the water and drop in the pasta. Stir to separate the strands and cook until just under al dente, tender but firm to the bite, stirring occasionally with a wooden spoon. Drain the pasta in a colander, reserving 1/2 cup (120 milliliters) of the pasta cooking water.

Meanwhile, in a large sauté pan over medium heat, combine 4 tablespoons (60 milliliters) of olive oil, 1 clove garlic, pepper flakes, and the parsley sprig and simmer until the garlic turns lightly golden and begins to softens. Raise the heat slightly and add the mussels to the pan. Cover the pan and sauté until the shells open, about 2 to 4 minutes, shaking the pan occasionally. Remove the pan from the heat.

Using a fine-mesh sieve, strain the mussels over a bowl to get rid of any sand and grit, reserving the juices. Discard the unopened shells, garlic, and the parsley sprig. In a medium bowl, set aside a few mussels in their shells to garnish the plates when serving. Remove the meat from the remaining shells and place in the bowl. Discard the shells.

In the same pan over moderate heat, combine the remaining olive oil and garlic and saute' until the garlic turns lightly golden and begins to soften. Add the tomatoes to the pan and stir occasionally for about 2 minutes or until the tomatoes begin to soften.

Pour in the mussel juice and cook for about 5 to 6 minutes or until the sauce has reduced a bit and the tomatoes have softened. Stir in the reserved mussels and parsley and cook for another minute or so longer just to allow the flavors a chance to meld together.

Discard the garlic. Add the pasta to the pan and toss until the pasta is al dente and well coated with the sauce, about 2 minutes, adding a little more pasta cooking water, if more liquid is needed.

Divide the pasta among the plates and top with the mussels in their shells. Drizzle with a little extra-virgin olive oil and garnish with parsley. Serve immediately.