Cut chicken into 3-inch pieces. Pat dry, season, and dredge in flour.
Heat olive oil in a large skillet over medium. Sear chicken 3 minutes per side, in batches. Set aside.
Add mushrooms to pan and cook until browned (5-7 minutes). Add garlic and cook 2 more minutes.
Season and pour in wine and stock. Scrape browned bits and reduce by half.
Return chicken to pan. Add spinach, cover, and cook 2-3 minutes to wilt.
If sauce is thin, stir in cornstarch slurry and simmer 2 minutes to thicken.
Turn off heat, stir in butter, adjust seasoning. Serve hot.
