Heat olive oil over medium heat in a large skillet. Add chicken and season with salt and pepper. Cook until browned on all sides and cooked through. Remove chicken and set aside.

Add onion to the skillet and cook until softened, about 3 minutes. Stir in the garlic and cook for another minute.

Return the chicken to the skillet. Add sour cream, chicken broth, thyme, and Dijon mustard. Stir and bring to a simmer.

Reduce heat to low and cook for a further 5 minutes, or until the sauce has thickened slightly. Season with additional salt and pepper, as needed.

Serve hot over cooked pasta, rice, or mashed potatoes. Garnish with fresh parsley, if desired.