In a food processor, combine the parsley, olives, cornichons, pickled onions, capers and olive oil and process until very finely chopped.
Preheat a grill pan or panini press. Arrange the bread on a work surface and spread 6 of the slices with the tapenade. Top with the Gruyère. Close the sandwiches with the remaining bread and put them on the hot grill pan or panini press. Cook over moderate heat until the bread is toasted and the cheese is melted, about 5 minutes. Transfer the panini to a work surface, cut in half and serve.