Preheat the oven to 400°F (200°C).
In a baking dish, place the chicken breasts and drizzle with olive oil.
In a small bowl, combine thyme, rosemary, garlic powder, salt, and pepper. Rub the spice mixture evenly over each chicken breast.
Bake the chicken in the preheated oven for 25-30 minutes, or until the chicken is cooked through (internal temperature reaches 165°F or 74°C).
While the chicken is baking, fill a pot with water and add the potato chunks. Bring to a boil over medium-high heat and cook for about 15-18 minutes or until the potatoes are tender.
Drain the potatoes, then return them to the pot. Add minced garlic, milk, butter, salt, and pepper.
Mash the potatoes using a potato masher or electric mixer until smooth and creamy. Adjust seasonings to taste.
Once the chicken is done, remove it from the oven and let it rest for 5 minutes before slicing.
Serve the sliced chicken breast alongside a generous portion of garlic mashed potatoes.