Cook the Brussels Sprouts in 1/2 cup of chicken broth; cover and set aside. Spray a small skillet with vegetable cooking spray and add the oil. Saute the onion in the oil then remove from the heat and slowly add the 1 cup of chicken broth. Sir in the mustard and pepper. Return to the heat and dissolve the cornstarch in the milk then add to the skillet mixture. Stirring constantly, cook until the sauce is smooth and thickened, about 5 minutes. Pour the mustard sauce over the cooked Brussels Sprouts and stir to coat. Serve at once.