Pat the chicken dry with paper towels. If the chicken breasts are thick, slice them horizontally to create thinner cutlets. Season both sides with salt, black pepper, and thyme.

Heat the olive oil and butter in a large skillet over medium-high heat. Add the chicken and sear for 4–5 minutes per side, or until golden brown. Transfer the chicken to a plate and set aside.

Add the sliced mushrooms to the same skillet. Cook for 5–7 minutes, stirring occasionally, until the mushrooms are tender and lightly browned.

Add the chopped shallot and minced garlic. Cook for 1 minute, stirring frequently.

If using white wine, pour it into the skillet and scrape the browned bits from the bottom. Allow it to simmer for 1–2 minutes.

Pour in the chicken broth and heavy cream. Add the Dijon mustard, if using, and stir to combine.

Add the Brie pieces and reduce the heat to medium-low. Simmer gently for 3–5 minutes, stirring occasionally, until the Brie melts and the sauce becomes creamy.

Return the chicken to the skillet and spoon the mushroom and Brie sauce over the top.

Cover and simmer for 5–8 minutes, or until the chicken is fully cooked and reaches an internal temperature of 74°C.

Garnish with chopped fresh parsley and extra black pepper. Serve warm with mashed potatoes, rice, pasta, vegetables, or crusty bread.