Season chicken with salt, pepper, and thyme. Heat oil in a large skillet over medium-high heat.
Sear chicken 3–4 minutes per side until golden and almost cooked through; transfer to a plate.
Add onions and mushrooms to skillet; cook 5 minutes until softened and browned.
Stir in garlic, then pour in chicken broth, scraping up browned bits.
Return chicken to skillet, add spinach, and simmer 2–3 minutes until wilted and chicken reaches 165°F.
Stir in Greek yogurt if using, adjust seasoning, and squeeze lemon over before serving.