Perfect Pesto Pasta

8 ounces short pasta, such as rigatoni2 teaspoons olive oil, plus more as needed1 cup chopped onion8 ounces sliced button mushrooms#189; cup grape tomato halves (see note)2 tablespoons Classic Basil Pesto, recipe follows (see note), or more to taste#189; cup shaved parmesan cheeseCook the pasta according to package directions.Meanwhile, heat the oil over medium heat in a 12-inch nonstick skillet. Peel and coarsely chop the onion, adding it to the skillet as you chop. Add the mushrooms to the skillet, and cook until the liquid is released, about 5 minutes. Stir frequently. Meanwhile, cut the tomatoes in half and set aside. When the mushrooms are done, remove the skillet from heat and set aside.Drain the cooked pasta into a colander, shaking to remove as much water as possible, and pour the pasta back into the cooking pot. Add all vegetables to the pot with the pasta. Add the pesto and parmesan, and stir and toss well to coat the pasta. Add a bit more olive oil, 1 teaspoon at a time, if needed, to keep the pasta from clumping together. Serve at once.Serves four.Cook's note: Cherry tomatoes can be used. Commercially prepared pesto can be used. If using homemade pesto that has been frozen, let it sit on the counter to thaw while cooking pasta or place in refrigerator overnight.@Recipe nutrition:Nutrition values per serving: 357 calories (30 percent from fat), 12 g fat (4 g saturated), 49 g carbohydrates, 3.5 g fiber, 15 g protein, 13 mg cholesterol, 235 mg sodium.