In a large cast iron skillet, heat the olive oil over medium-high heat. Place the apples cut side down, and cook, turning occasionally, for 5-8 minutes, until golden brown. Add the sausages to the skillet with the apples, and prick with a fork. Cook, turning occasionally, for 10-12 minutes, until browned. Stir in the white wine and vinegar, and bring to a boil. Reduce heat to medium-low. Simmer until the pan sauce thickens, about 4 minutes. Fold in the watercress and season with salt and pepper.