Cook the onion for 5 minutes in the olive oil in a medium frying pan over medium heat.
Add the tomatoes and thyme then season with salt and pepper. Add the white wine and cook for 5 minutes.
Add the mussels (in several batches if the frying pan is crowded) and cover for 3 minutes. If the mussels are not opened, cover and cook for one or 2 more minutes. Remove any unopened mussels and discard. Serve immediately.