Add the cubed russet potatoes to a large pot and cover them with cold water. Bring to a boil and cook until fork-tender, about 15 minutes. Drain the water, then return the potatoes to the pot. Add butter, heavy cream, milk, and salt. Mash the potatoes until they’re smooth and creamy. Cover the pot and set aside to keep warm.
Season both sides of the chicken breasts with salt, pepper, and garlic powder. Heat butter and olive oil in a skillet over medium-high heat. Once hot, add the chicken and sear for about 6-7 minutes on each side, or until golden brown. Remove the chicken from the skillet and set aside.
To the same skillet, reduce the heat to medium and melt butter. Add the minced garlic and cook until fragrant, about 1 minute. Whisk in the flour and cook for another minute. Gradually pour in the chicken broth and white wine while whisking to incorporate. Stir in the Dijon mustard, salt, and pepper. Whisk in the heavy whipping cream and simmer for 1-2 minutes until the sauce thickens to a gravy-like consistency.
Preheat your oven to 375°F. Add the seared chicken back to the skillet, spooning some sauce over the top. Transfer the skillet to the oven and bake for 15-18 minutes, or until the chicken is fully cooked.
Spoon the mashed potatoes onto plates, place the chicken on top, and generously drizzle with the creamy Dijon sauce. Garnish with parsley and serve warm. Enjoy!