In a small bowl, combine cra¨me fraa®che, wasabi, scallion, and lemon juice. Lay salmon slices horizontally on a cutting board. Cut in half vertically. Spread 1 teaspoon wasabi cream down center (vertically).
Place watercress in center vertically, stem at bottom, so that the leaves poke out from the top. Roll horizontally from wide end to thin, gently but tightly. Trim greens if they poke out of the bottom. Repeat with remaining salmon.
Arrange upright on a serving platter. Serve immediately, or cover and refrigerate 30 minutes or more (up to 6 hours).