Cook linguine according to package directions. Meanwhile, in a large skillet, saute the onions and garlic in butter and oil until garlic is tender. Sprinkle chicken with salt and pepper; add to skillet. Cook for 3-4 minutes on each side or until a meat thermometer reaches 170°. Remove chicken and keep warm.
In the same skillet, combine the tomato, lemon juice and parsley; heat through. Drain linguine; toss with tomato mixture. Top with chicken and sprinkle with Parmesan cheese. Yield: 2 servings.