Step 1Bring a large pot of water to a rapid boil. Add enough salt to the water until it tastes salty. Blanch the watercress until it is very soft, then immediately drain and transfer to an ice bath to stop the cooking process and chill completely.
Step 2Drain the watercress well, gently wringing out most of the water. Weigh about 11 ounces (300 grams) of the watercress and transfer to a high-speed blender or food processor. Discard the rest, or save for another use. Blend the watercress on high speed until it is very smoothly pureed.
Step 3In a small saucepan, bring the cream to a boil. Immediately remove from heat and drizzle into the blender with the pureed watercress. Add 1 1/4 teaspoons kosher salt, and then the xanthan gum, to the mixture while blending on high speed. Continue to blend the mixture until it is hot, or remove to a saucepan over medium heat, stirring frequently, until hot.
Step 4Strain the mixture into a 1/2-liter-capacity iSi siphon and charge with 2 cream chargers. Shake vigorously for about 10 seconds and test the mousseline for consistency. Reserve the mousseline in a warm location until ready to plate.