Quick Vegetable Mac 'n' Cheese

8 ounces shells or elbow pasta 3 cups mixed vegetables such as broccoli and cauliflower florets, sliced carrots, and chopped bell pepper 11⁄4 cups fat-free milk 2 tablespoons cornstarch 2 tablespoons extra-virgin olive oil 1 medium onion, chopped 1 cup (4 ounces) shredded reduced-fat sharp Cheddar cheese 2 ounces reduced-fat cream cheese   1. Prepare the pasta according to package directions, adding the vegetables to the water during the last 5 minutes of cooking time. Drain and place in a serving bowl.   2. Meanwhile, in a measuring cup, whisk together the milk and cornstarch. Heat the oil in a medium saucepan over medium heat. Add the onion and cook, stirring frequently, for 5 minutes. Add the milk mixture and bring to a boil, stirring constantly. Reduce the heat to medium-low and add the Cheddar and cream cheese. Stir until melted. Pour over the pasta mixture and toss to coat well.