Prep the chicken. Pat chicken breasts dry and season both sides with 1 teaspoon salt and 1/2 teaspoon black pepper. If they’re thick, lightly pound to an even thickness for faster, more even cooking.

Sear for flavor. Heat olive oil in a large skillet over medium-high heat. Add chicken and sear 4–5 minutes per side until golden. Transfer to a plate; they’ll finish cooking in the sauce.

Cook the mushrooms. Lower heat to medium. Add butter and mushrooms with a pinch of salt. Cook, stirring occasionally, 6–8 minutes until browned and their moisture has cooked off.

Sweat the aromatics. Add diced onion and cook 3–4 minutes until soft and translucent. Stir in garlic and thyme; cook 30 seconds until fragrant.

Build the sauce base. Pour in chicken stock, scraping up any browned bits from the pan. Stir in Dijon and Worcestershire. Simmer 3–4 minutes to reduce slightly.

Make it creamy. Reduce heat to low. If using Greek yogurt, whisk it in off the heat a spoonful at a time to prevent curdling, then return to low heat. If using light cream, pour it in and simmer gently. For a thicker sauce, stir in the cornstarch slurry and simmer 30–60 seconds until glossy.

Finish the chicken. Nestle chicken back into the pan with any juices. Simmer gently 4–6 minutes, turning once, until the chicken reaches an internal temperature of 165°F (74°C).

Taste and adjust. Add salt and pepper as needed. If the sauce tastes flat, add another small splash of stock or a squeeze of lemon to brighten.

Serve. Sprinkle with chopped parsley. Spoon sauce over the chicken and serve with your favorite sides.