They are made in small, individual serving-size Dariole moulds.
The inside of the moulds is coated with a savoury jelly, based on either a fish or meat stock (depending on whether the main item on the plate is meat or fish.) Into this jelly, you press decorative pieces of food such as slices of truffle, hard boiled egg white, tongue or fresh tarragon leaves.
Then, you fill the space remaining with a filling made from cooked vegetables (small ones such as peas, or diced larger vegetables such as green beans, carrots, artichokes, tips of asparagus, etc), bound with a mayonnaise mixed with jelly.
Once filled, you glaze the top with a layer of the jelly, then chill them in the refrigerator to set them.