Heat chicken according to package directions; hold warm.

While chicken is cooking, heat 2 tablespoons oil in large skillet over medium heat. Add mushrooms, and sauté turning occasionally, until golden brown, about 4-5 minutes. Remove mushrooms from pan; hold warm.

Add remaining 1 tablespoon oil to skillet and heat over medium heat. Add onion and garlic; sauté for 3 minutes or until tender then add spinach and lemon juice; continue cooking while stirring until spinach softens and warms through. Season to taste with salt and pepper.

To serve, evenly divide spinach mixture on 4 plates. Top with chicken and spoon mushrooms over chicken. Sprinkle shaved Parmesan cheese, if desired.