Prepare the Chicken: Pat the chicken thighs dry with paper towels. Season generously with salt and pepper.

Sear the Chicken: Heat olive oil and butter in a large, heavy-bottomed skillet over medium-high heat. Add the chicken thighs, skin-side down, and sear for 5-7 minutes, or until the skin is golden brown and crispy. Don't overcrowd the pan; work in batches if necessary. Flip the thighs and sear for another 3-5 minutes. Remove the chicken from the skillet and set aside.

Sauté the Vegetables: Reduce the heat to medium. Add the sliced onion to the skillet and cook for 5-7 minutes, or until softened and translucent. Add the minced garlic and sliced mushrooms and cook for another 3-5 minutes, until the mushrooms are tender and have released their moisture.

Create the Gravy: Stir in the dried thyme, rosemary, and red pepper flakes (if using). Sprinkle the flour over the vegetables and cook for 1-2 minutes, stirring constantly, to create a roux.

Deglaze and Simmer: Gradually pour in the chicken broth, scraping up any browned bits from the bottom of the skillet. Bring the mixture to a simmer, stirring occasionally, until the gravy has thickened slightly, about 5-7 minutes.

Smother the Chicken: Return the chicken thighs to the skillet, nestling them into the gravy. Reduce the heat to low, cover the skillet, and simmer for 30-40 minutes, or until the chicken is cooked through and tender. The internal temperature should reach 165°F (74°C).

Finish and Serve: Stir in the heavy cream. Season with salt and pepper to taste. Garnish with fresh parsley. Serve hot over mashed potatoes, rice, or your favorite side dish.