Step 1

In a large pot, heat the oil over moderately low heat. Add the onion and garlic and cook, stirring occasionally, until the onion is translucent, about 5 minutes. Stir in the parsley, tomatoes, thyme, and red-pepper flakes. Reduce the heat and simmer, partially covered, for 25 minutes, stirring occasionally.

Step 2

Discard any mussels that have broken shells or that don't clamp shut when tapped. Add the mussels to the pot. Cover; bring to a boil. Cook, shaking the pot occasionally, just until the mussels open, about 3 minutes. Remove the open mussels. Continue to boil, uncovering the pot as necessary to remove the mussels as soon as their shells open. Discard any that do not open.

Step 3

Stir the black pepper into the broth. Taste the broth and, if needed, add salt. Ladle the broth over the mussels and serve with the garlic toast.