Place the peeled and cubed potatoes in a large pot of salted water. Bring to a boil over high heat, then reduce to medium-high and cook for 15-20 minutes until fork-tender.

While the potatoes cook, season both chicken breasts evenly with garlic powder, salt, and black pepper on both sides. Let them sit at room temperature for 5-10 minutes.

Heat 1 tablespoon olive oil and 1 tablespoon butter in a large skillet over medium-high heat. Once the butter stops foaming, add the seasoned chicken breasts. Cook for 6-7 minutes per side until golden-brown and internal temperature reaches 165°F. Remove from skillet and tent with foil.

Drain the potatoes thoroughly and return them to the hot pot. Add 2 tablespoons butter and the warmed milk. Mash until creamy and smooth. Season with salt and pepper to taste. Keep warm with lid on.

In the same skillet used for chicken, add 1 tablespoon olive oil if needed. Add the sliced onion and cook for 2-3 minutes until softened and translucent.

Add the sliced bell peppers, mushrooms, and minced garlic to the skillet. Sauté for 5-7 minutes, stirring occasionally, until vegetables are tender-crisp and mushrooms have browned. Season with salt and pepper to taste.

To serve, place a generous scoop of mashed potatoes in the center of each plate. Top with chicken breast (sliced if desired) and arrange the sautéed vegetables around the plate. Finish with remaining butter melted over chicken and garnish with fresh parsley.