Season both sides of the chicken breasts with salt and pepper.
Heat olive oil in a large skillet over medium heat. Add chicken and cook 6–7 minutes per side, or until golden and cooked through. Remove from skillet and set aside.
In the same skillet, melt butter and sauté the onion and garlic until fragrant and translucent, about 2–3 minutes.
Add the spinach and cook until wilted, about 2 minutes.
Stir in the heavy cream, Parmesan, and red pepper flakes. Simmer until the sauce thickens slightly.
Return the chicken to the skillet and spoon the sauce over the top. Let simmer for 2–3 more minutes until heated through.
Serve warm with your favorite side dish.