In a large nonstick skillet, saute onion in 1 tablespoon oil until tender. Add half of the garlic; saute 1 minute longer. Transfer to a small bowl; set aside.
In the same skillet, cook chicken in 1 tablespoon oil over medium heat for 5-7 minutes or until juices run clear. Remove from the heat. Add 1 tablespoon pesto; toss to coat. Stir in half of the onion mixture. Transfer to a bowl; keep warm.
Cook pasta according to package directions. Meanwhile, add mushrooms and butter to the skillet; cover and cook over medium heat for 5-7 minutes or until tender. Reduce heat to low. Add spinach; cover and cook for 2-3 minutes or until wilted. Drizzle with 1 tablespoon vinegar and remaining onion mixture; cook and stir until heated through. Keep warm.
In a small saucepan, combine 1 tablespoon pesto and remaining vinegar and garlic. Cook and stir until heated through.
Drain pasta; toss with remaining oil and pesto. On two plates, form pasta into a circle, leaving a hole in the center. Place mushrooms with spinach mixture in the center; top with chicken mixture. Drizzle with garlic pesto mixture. Sprinkle with Romano and Parmesan. Yield: 2 servings.