Leave a bit of moisture on the spinach and watercress after cleaning. Place in a saucepan; stir over medium heat until leaves are wilted, about 3 minutes. Drain well and squeeze in a kitchen towel to release moisture. Chop coarsely and set aside.
Preheat oven to 400-degree F. Butter or coat with non-stick cooking spray a 2-quart round soufflé dish.
Melt the butter in a heavy saucepan over low heat. Add the flour and whisk until smooth. Next, whisk in the milk, nutmeg, salt and pepper. Raise heat to medium and cook, stirring constantly, for 2 minutes or until the mixture is thick and slightly boiling. Add the goat cheese and whisk until smooth. Remove from heat and cool slightly.
Beat egg yolks in a medium bowl until lemon-colored and whisk a 1/4 cup cooled sauce into them. Whisk the egg yolk mixture a bit at a time into the cool sauce. Stir in the spinach-watercress mixture along with parsley and chives.
With an electric mixer, begin to beat the egg whites with the lemon juice until frothy, then continue beating until stiff but not dry.
With a rubber spatula, gently fold one-quarter of the egg whites into the sauce. Quickly and gently fold the remaining whites into the sauce until well combined. Transfer to the prepared soufflé dish.
Bake for 30 minutes or until soufflé is golden and puffed, yet still moist.