Season the chicken breasts with salt and pepper on both sides. In a large skillet, heat the olive oil over medium-high heat. Add the chicken breasts and cook for about 6-7 minutes on each side, or until golden brown and cooked through. Remove the chicken from the skillet and set aside.
In the same skillet, add the chopped onion and sauté for 3-4 minutes until softened. Add the minced garlic and cook for an additional 1 minute until fragrant.
Pour in the chicken broth, scraping the bottom of the skillet to deglaze. Stir in the heavy cream, Dijon mustard, thyme, and lemon juice. Bring the sauce to a simmer and let it cook for about 5 minutes until slightly thickened.
Return the chicken to the skillet, coating it with the creamy sauce. Let it simmer for another 2-3 minutes to heat through. Garnish with chopped parsley before serving.
For the mashed potatoes, boil the cubed potatoes in a large pot of salted water until tender, about 15-20 minutes. Drain and return to the pot. Add the milk, butter, salt, and pepper. Mash until smooth and creamy.
Serve the creamy Dijon chicken over a bed of mashed potatoes, drizzling extra sauce on top.