Preheat oven to 200C / 400F.
Using a mallet, pound chicken breasts into 2-inch thick slices. Season both sides with salt and pepper.
Meanwhile, heat 2 tablespoons butter in a large oven proof skillet over high heat. Add chicken. Sear for about 2-3 minutes on each side or until golden brown. Remove from pan. Set aside.
Reduce heat to medium-high.
Add butter and olive oil.
Add garlic and lemon zest. Sauté, stirring, for about 1 minute or until fragrant.
Add chicken stock or broth, cream and lemon juice. Stir to combine. Season with salt. Bring to a boil.
Add spinach and parsley. Stir to combine. Simmer for about 3-5 minutes or until the spinach is wilted and sauce starts to thicken.
Return chicken to the pan.
Transfer to the oven. Cook for about 25-30 minutes. The internal temperature needs to be 74C or 165F.
Serve immediately.