1. Season the chicken with salt and pepper. Heat olive oil in a large skillet over medium heat and cook chicken until golden and cooked through (about 4-5 minutes per side). Remove and set aside.
2. In the same skillet, melt butter and sauté onions and mushrooms until softened. Add garlic and cook for another minute.
3. Sprinkle flour over the mushroom mixture and stir well. Gradually whisk in chicken broth until the gravy thickens.
4. Stir in heavy cream (if using) and season with salt and pepper to taste.
5. Return the chicken to the skillet and simmer for 5 minutes, allowing the flavors to meld.
6. Serve hot over mashed potatoes, garnished with thyme or parsley.