PREPARE pasta according to package directions, reserving ½ cup cooking water. While pasta is cooking, prepare leek mixture.
HEAT oil in large skillet over medium-high heat. Add leek; cook, stirring frequently, until softened and golden. Stir in peas; cook for 1 minute or until heated through. Season with salt and pepper.
ADD reserved pasta cooking water, thyme leaves and cheese; stir in gently. Toss pasta with leek mixture. Season with salt and pepper.