Slice chicken breasts horizontally to create 4 thinner cutlets. Season both sides generously with salt and pepper.
Heat olive oil in a large skillet over medium-high heat. Add chicken cutlets and cook for 3-5 minutes per side, or until golden brown and cooked through. Remove chicken from skillet and set aside on a plate.
Melt butter in the same skillet over medium heat. Add diced onion (or shallots) and cook until softened, about 3-4 minutes. Add minced garlic and cook for 1 minute until fragrant.
Pour in white wine (or chicken broth) and bring to a simmer, scraping up any browned bits from the bottom of the pan. Let the liquid reduce by half, about 2-3 minutes.
Stir in heavy cream and Parmesan cheese. Bring to a gentle simmer, stirring until the cheese is melted and the sauce is smooth and slightly thickened.
Add fresh spinach to the skillet and cook, stirring, until it wilts completely, about 2-3 minutes.
Return the cooked chicken cutlets to the skillet, nestling them into the sauce. Simmer for another 1-2 minutes to heat through.
Taste and adjust seasonings if necessary. Garnish with fresh chopped parsley and serve immediately.
Notes
For a richer flavor, use chicken thighs instead of breasts.
If you don’t have white wine, chicken broth is a great substitute.
Freshly grated Parmesan cheese will give the best flavor and texture to the sauce.
Serve with pasta, rice, or crusty bread to soak up the delicious sauce.