1/2 cup dry white wine or reduced-sodium chicken broth
Pound chicken breasts with a meat mallet to 1/2-in. thickness; sprinkle with salt and pepper. In a large skillet, heat oil over medium heat; cook chicken until no longer pink, 5-6 minutes per side. Remove from pan; keep warm.
Add mushrooms and onion to pan; cook and stir over medium-high heat until tender and lightly browned, 2-3 minutes. Add garlic; cook and stir 30 seconds. Add wine; bring to a boil, stirring to loosen browned bits from pan. Cook until liquid is slightly reduced, 1-2 minutes; serve over chicken.