1 Tbsp. olive oil
1 c. loosely packed fresh-leaf parsley leaves, coarsely chopped
2 whole boneless, skinless chicken breasts split
Salt and freshly ground pepper
1 small onion, chopped
10 white mushrooms, trimmed and cut into 3/4" pieces
1 c. dry white wine
2 Tbsp. balsamic vinegar
1 Tbsp. butter
4 handfuls (8-10 oz) fresh baby spinach, stems removed.

1. Heat oven to 400 degrees. Heat olive oil in a large skillet over medium heat. Press a scant Tbsp. parsley into each chicken breast. Season with salt and pepper. Place breasts in skillet, cook until golden, about 8 minutes per side.
2. Remove chicken from skillet; transfer to a roasting pan. Transfer pan to oven until chicken cooks through, 15-20 minutes.
3. Meanwhile, return skillet to stove. Add onion; cook, stirring until just golden, about 2 minutes. Add mushrooms, cook until soft, about 5 minutes. Add wine; scrape skillet's bottom to release cooked -on juices. Cook until wine is almost evaporated, about 5 minutes. Add vinegar; cook 1 minute more. Stir in remaining parsley; season with salt and pepper, Remove from heat; stir in butter.
4. Remove chicken from oven. Place spinach on plates, slice each breast into sixths; arrange over spinach. Spoon mushroom mixture over chicken, drizzling juices over spinach and serve.