Put a quarter each of the watercress, scallops, and shallot in each of four 6-ounce ramekins and sprinkle with salt and pepper. Beat the eggs lightly and combine with the stock, soy, and sesame oil, if using.
Put the ramekins in a baking pan or skillet and fill them with the egg mixture. Add boiling water to reach about halfway up the sides of the ramekins and turn the heat to high. When the water returns to the boil, turn the heat to low and cover tightly.
Simmer for 15 minutes, then check. The custards are done when they have set and are no longer watery but are still quite jiggly. Immediately remove them from the water and serve hot or at room temperature.
