In a medium bowl, combine the minced garlic, chopped rosemary, chopped thyme, olive oil, salt, and pepper. Add the chicken thighs, coating them well in the herb mixture. Let it marinate for at least 30 minutes (or up to overnight in the refrigerator for deeper flavor).
Preheat a large skillet over medium heat. Once heated, add the chicken thighs. Sear for about 5-6 minutes on each side until they are golden brown and cooked through. Remove the chicken from the skillet and set aside.
In the same skillet, pour in the chicken broth, scraping the bottom of the pan to deglaze it. Allow it to simmer for 2-3 minutes. Then add the heavy cream, stirring well. Allow it to cook until slightly thickened, about 2-3 minutes. Return the chicken to the skillet, spooning the sauce over it. Simmer for an additional 2 minutes.
While the chicken is finishing, bring a large pot of salted water to a boil. Add the chopped potatoes and cook until tender, around 15-20 minutes. Drain the potatoes well and return them to the pot.
Add butter, milk, and a pinch of salt and pepper to the drained potatoes. Mash until smooth and creamy. Adjust seasonings to taste.
Plate the creamy mashed potatoes and top with the garlic herb chicken. Drizzle some of the rich sauce over the chicken and mash for added flavor.