Using a sharp knife, slice each chicken breast diagonally into 1/2-inch thick pieces. Pound the pieces in a plastic bag until they are 1/4-inch thick.
Heat oil and spray in a large non-stick skillet. Add half the chicken. Cook over medium heat until barely done, about 1 minute per side; they will not be white in all places.
Transfer the chicken pieces to a plate and repeat with the rest of the chicken, adding spray if necessary.
Using the same skillet and lowering the heat, carefully add wine, sugar substitute and lemon juice to the skillet, season with salt and pepper. Raise the heat and bring to a boil. Remove from heat and allow to cool.
Remove 1/2 cup of the cooled lemon juice mixture and stir in flour until dissolved. Return this mixture to the skillet.
Return chicken to skillet and cook, stirring constantly, until the slices are completely white, about 5 minutes.
Line a platter with the watercress and radicchio, and arrange the chicken slices on top. Garnish with the remaining sauce and lemon slices and serve immediately.