1. Place the chopped potatoes in a large pot and cover with cold water. Add salt, bring to a boil, and cook until fork tender.

2. Drain the potatoes well, then return them to the warm pot for 1 minute so excess steam can escape.

3. Add warm milk, butter, salt, and black pepper to the potatoes. Mash until creamy and smooth, then cover to keep warm.

4. Pat the chicken dry and season both sides with Italian seasoning, salt, and black pepper.

5. Heat olive oil and 1 tablespoon butter in a large skillet over medium high heat. Sear the chicken for 4 to 5 minutes per side until golden, then transfer to a plate.

6. Reduce the heat to medium. Add the remaining butter to the same skillet, then stir in the garlic and cook for about 30 seconds until fragrant.

7. Pour in the chicken broth and scrape up the browned bits from the bottom of the skillet.

8. Stir in Dijon mustard and heavy cream. Simmer for 3 to 4 minutes until the sauce begins to thicken.

9. Return the chicken and any juices to the skillet. Spoon sauce over the chicken and simmer until the chicken reaches 165°F.

10. Stir in Parmesan cheese if using, then taste and adjust seasoning.

11. Spoon mashed potatoes onto plates, top with chicken, and ladle the garlic herb sauce over everything.

12. Garnish with fresh parsley and serve warm.