Cut the chicken into roughly 3-inch long pieces. Pat the chicken dry and season well with salt and pepper on both sides. Dredge the chicken in flour on all sides, shaking off excess.

Heat a large skillet over medium heat. Add 1-2 tablespoons of olive oil to the pan. Once hot, add the chicken pieces. Do not overcrowd the pan. Sear the chicken for 3 minutes per side or until cooked through, then place on a plate lightly tented with foil. Work in batches, adding more oil as needed, and set all chicken aside.

Add the mushrooms to the pan and cook until browned, about 5-7 minutes. Once browned, add the garlic to the pan and cook for an additional 2 minutes.

Season the mushrooms with salt and pepper. Add the wine and chicken stock to the pan, turning the heat to high. Scrape all the brown bits off the bottom of the pan with a wooden spoon. Cook until the liquid reduces by half, about 3-5 minutes, then reduce the heat to medium.

Add the chicken and spinach to the pan. Cover and cook for 2-3 minutes to wilt the spinach and warm the chicken through.

If the sauce is too thin, add half of the optional cornstarch mixture to the pan and cook over medium-high heat until thickened, about 2 minutes. If still too thin, add the remaining mixture. Once satisfied with the sauce's consistency, turn off the heat and remove the pan from the heat.

Taste test the sauce and adjust salt and pepper if needed. Mix in the cubed butter and stir until melted. Serve with crusty bread to soak up the sauce. Enjoy!