Cook the onion and thyme leaves in the oil, in a large pan. Just as they are beginning to color, add the mussels. Cook over high heat, stirring often so that those on top get to the bottom. When they have opened, remove them from their shells. Use a mortar and pestle to make a paste of the garlic and basil leaves. Add the oil and beat well, then add the vinegar, salt and pepper. Pour this sauce over the mussels. Let them marinate for 30 minutes in the refrigerator. Serve on a bed of salad.
