1. Season chicken breasts with salt and pepper.
2. Heat olive oil in a large skillet over medium-high heat.
3. Sear chicken breasts for 3-4 minutes per side, until golden brown. Remove from skillet and set aside.
4. In the same skillet, sauté onion until softened. Add mushrooms and garlic, cook until softened.
5. Pour in chicken broth and bring to a simmer, scraping up any browned bits from the bottom of the pan.
6. Reduce heat to low, stir in heavy cream and Parmesan cheese. Simmer until sauce thickens slightly.
7. Stir in butter until melted. Return chicken to skillet, coating with sauce.
8. Simmer for 5-7 minutes, or until chicken is cooked through.
9. Garnish with fresh parsley and serve.