Prep the chicken: Pat chicken dry and slice thick breasts horizontally into cutlets if needed for even cooking. Season both sides with salt and pepper.

Heat the pan: Warm 1 tablespoon olive oil in a large skillet over medium-high heat until shimmering.

Sear the chicken: Add chicken in a single layer. Sear 3–4 minutes per side until golden and just cooked through (internal temp 165°F). Transfer to a plate and tent with foil.

Sauté aromatics: Lower heat to medium. Add another drizzle of oil if the pan looks dry. Add onion and a pinch of salt; cook 3–4 minutes until softened. Stir in garlic for 30 seconds.

Brown the mushrooms: Add mushrooms and a small pinch of salt. Cook 5–7 minutes, stirring occasionally, until browned and most liquid evaporates.

Build the sauce: Add thyme, chicken broth, lemon zest, and a squeeze of lemon juice. Bring to a lively simmer and scrape up browned bits from the pan.

Optional creaminess: Reduce heat to low. In a small bowl, whisk Greek yogurt with a tablespoon of warm sauce, then stir it into the skillet. Add mustard if using. Simmer 1–2 minutes to thicken slightly. Do not boil.

Return the chicken: Add chicken and any juices back to the skillet. Spoon mushrooms and sauce over the top. Simmer 1–2 minutes to warm through. Adjust salt, pepper, and lemon to taste. Add a pinch of red pepper flakes if you like heat.

Finish and serve: Sprinkle with parsley. Serve as is, or over cauliflower rice, zucchini noodles, or a small scoop of brown rice or quinoa.