Brush the mussels well under running water and de-bark them. Separate damaged specimens. Peel and chop the onion and garlic clove. Carefully clean the celery, cut away the ends of the stalks and cut into small pieces.
Add the mussels to the tomato soup form and let it bubble up with the lid closed. Continue until the mussels have opened. Only now season the soup with salt and freshly ground pepper.
While the mussels are cooking, heat the olive oil (2) and toast the bread slices in it until golden brown.
Coarsely chop the basil. Arrange the bread slices in deep plates. Spread the basil evenly over the bread and arrange the mussels on top.