Wash the watercress. Combine it with the spring mix, celery leaves, chicken stock and lemon juice in a blender or food processor (the blender works better) and process into a rough puree.
In a saucepan, saute leek in the butter until clear but not browned. Add the wine and bring to a boil. Cook until the liquid is almost gone. Lower the heat to medium-low.
Add the flour, salt, pepper, cayenne, and nutmeg to the pan. Cook while stirring until the texture changes (as it would when making a roux), but don't allow to brown. Whisk in the warm milk and cook until the mixture is smooth and thick.
Blend in the puree of greens. Add a little more chicken stock or water if necessary to give the soup the proper consistency. Add the shrimp, and cook over medium-low heat until the pot begins to simmer and the shrimp are warmed through. Serve garnished with green onions. Season with Tabasco to taste.