Pat the chicken thighs dry and season them with salt and pepper.
Heat olive oil and butter in a large skillet over medium heat.
Sear the chicken thighs skin-side down until golden brown, about 5-6 minutes, then flip and cook another 5 minutes. Remove and set aside.
Add onions and garlic to the skillet and cook until softened.
Stir in Dijon mustard and chicken stock, scraping up any browned bits.
Return the chicken to the pan and let it simmer for 20 minutes, covered.
Stir in the heavy cream and thyme, then simmer uncovered for 5-7 minutes until the sauce thickens.
Taste and adjust seasoning. Garnish with parsley before serving.