Serve with fish—for example, the salmon. Or use it for a seafood cocktail. In any case, you’ll find a way to use this.
In a blender, process the watercress with the egg yolks, lemon juice, and miso until smooth, adding a tiny bit of salt and pepper if you wish. Miso is also salty, so beware.
Add the oil in a thin trickle, while pulsing the blender at the lowest speed. This way you’ll prevent the mayonnaise from curdling. When the mayonnaise begins to thicken, you can add the oil a bit more quickly. Taste whether the flavor is balanced: Sometimes you may want to add some more salt, or only some lemon juice.
Cover the miso mayo and put in the refrigerator until use.