Prep the chicken. Pat the chicken dry and season both sides with salt and pepper. For even cooking, lightly pound thicker breasts to an even thickness.
Sear for flavor. Heat olive oil in a large skillet over medium-high.
Add chicken and sear 3–4 minutes per side until golden. Remove to a plate; it will finish cooking in the sauce.
Soften the aromatics. Reduce heat to medium. Add the diced onion with a pinch of salt.
Cook 3–4 minutes, stirring, until translucent. Add garlic and cook 30 seconds until fragrant.
Deglaze. Pour in the chicken broth, scraping up any browned bits from the pan. Let it simmer 2 minutes to reduce slightly.
Build the creamy base. Lower heat to medium-low.
In a small bowl, whisk the yogurt, milk, Dijon, and whole-grain mustard (if using) until smooth. Stir this into the pan with the thyme.
Simmer gently. Return the chicken and any juices to the skillet. Spoon some sauce over the pieces.
Simmer on low for 5–7 minutes, or until chicken reaches 165°F at the thickest part.
Adjust thickness. If you want a thicker sauce, stir in the cornstarch slurry and simmer 1–2 more minutes. Keep the heat low to avoid curdling.
Brighten and finish. Stir in lemon zest, lemon juice, parsley, and tarragon (if using). Taste and adjust salt, pepper, and Dijon to your liking.
The sauce should be tangy, creamy, and balanced.
Serve. Plate the chicken with plenty of sauce. Garnish with extra parsley. Pair with a light side like steamed green beans or roasted broccoli.