Lightly beat the eggs, sour cream, and salt and pepper in a bowl. Stir in the watercress.
Melt the butter in a nonstick skillet, and swirl to coat the bottom of the pan. Add the eggs and cook over low heat, stirring constantly with a fork. Cook until the eggs are set and soft, 2½ to 3 minutes; do not overcook. Garnish each portion with a dollop of sour cream an