Preheat the oven to 500 degrees. Heat four 10-inch oven-proof dinner plates in the oven for 3 to 5 minutes. To ensure the success of the raclettes, the plates must be very hot.
When ready to serve, remove the plates from the oven. Working as quickly as possible, arrange four slices of cheese in the center of each plate, overlapping them slightly. The cheese should begin to sizzle once it comes into contact with the plate. At once, place the four plates on the floor of the oven. Let cheese melt in the oven for about 2 minutes, being careful not to let it brown. Remove from the oven, place a potato and one or two gherkins and pickled onions on the side of each plate and serve at once. Serve each plate on a service plate to prevent scorching the table.