Peel kohlrabi with a vegetable peeler and cut into 2-inch long julienne strips. Discard coarse stems from watercress. In a large heavy skillet, heat butter and reserved duck fat over moderately high heat until foam subsides and saute kohlrabi, stirring, until crisp-tender, 5 to 10 minutes. Add lemon juice and saute kohlrabi, stirring, 1 minute more. Remove skillet from heat and immediately stir in watercress. Season vegetables with salt and pepper.