Pound and season the chicken. If using breasts, lightly pound to even thickness (about 3/4 inch). Pat dry and season both sides with salt and pepper. This helps the chicken cook evenly and brown better.

Heat the pan. Set a large skillet over medium-high heat. Add 1 tablespoon olive oil and 1 tablespoon butter. When the butter foams, the pan is ready.

Sear the chicken. Add chicken in a single layer. Don’t crowd the pan. Cook 4–6 minutes per side until golden and nearly cooked through. Transfer to a plate and tent loosely with foil.

Sauté the mushrooms. Lower heat to medium. Add remaining butter and a drizzle of oil if the pan looks dry. Add mushrooms and a pinch of salt. Cook, stirring occasionally, until they release liquid and turn golden at the edges, 6–8 minutes.

Add aromatics. Stir in chopped onion and cook 2–3 minutes until softened. Add garlic and cook 30 seconds until fragrant. Don’t let it brown.

Deglaze. Pour in the chicken stock. Scrape up the browned bits with a wooden spoon. Simmer 2–3 minutes to reduce slightly.

Make it creamy. Stir in heavy cream and Dijon (if using). Simmer gently 2–3 minutes until the sauce begins to thicken. If you prefer a thicker sauce, add the cornstarch slurry and simmer another minute until glossy.

Return chicken to the pan. Nestle the chicken and any juices back into the sauce. Spoon sauce over the top. Simmer on low 3–5 minutes until the chicken is cooked through and the sauce coats a spoon. Aim for an internal temp of 165°F (74°C).

Finish and season. Stir in fresh herbs and a small squeeze of lemon for brightness. Taste and adjust salt and pepper. The sauce should be savory with a gentle tang.

Serve warm. Spoon over mashed potatoes, rice, buttered noodles, or polenta. Add extra herbs on top for color.