Melt the butter in a large pot over medium heat. Add the garlic, onion, ½ teaspoon salt, and ¼ teaspoon pepper. Cook until just tender, 4 to 5 minutes.
Add the wine and simmer until slightly reduced, 2 to 3 minutes. Stir in the tomatoes and olives.
Add the mussels and cook, covered, until they open, 3 to 4 minutes. Discard any that remain closed. Stir in the parsley.
Divide the mussels and cooking liquid among bowls. Serve with the bread.