NOTE: Master Class columns are intended for ambitious home cooks, and some of the recipes may call for special ingredients or equipment. When that is the case, we will provide an alternative method for the main element of the dish so it can be prepared even without this ingredient or equipment.
2 to 3 cups mousseline
This recipe requires a kitchen scale, xanthan gum and an iSi siphon. Xanthan gum can generally be found at health food stores and cooking supply stores. ISi siphons can generally be found at cooking and restaurant supply stores.
1 1/4 teaspoons kosher salt, plus extra for salting the blanching water
3 to 4 large bunches of watercress, washed, very large stems removed
1/2 cup (125 milliliters) heavy cream
1/2 teaspoon (1.2 grams) xanthan gum
1. Bring 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.
2. Drain 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.
3. In a small saucepan,
bring the cream to a boil. Immediately remove from heat and drizzle the puree into the blender with the watercress. Add 11/4 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.
4. Strain 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.