Preheat the oven to 425°F (220°C).
In a small bowl, mix together the kosher salt, black pepper, smoked paprika, and garlic powder.
Pat the chicken thighs dry with paper towels. Rub the olive oil evenly over the chicken thighs, then sprinkle the seasoning mixture evenly on all sides of the chicken.
Place the chicken thighs skin-side up on a rimmed baking sheet or in a roasting pan. Roast in the preheated oven for 35-40 minutes, or until the internal temperature reaches 165°F (74°C) and the skin is crispy.
While the chicken is roasting, place the peeled and cut potatoes and garlic cloves in a large pot and cover with cold water. Add a pinch of salt to the water.
Bring the water to a boil over high heat, then reduce to a simmer and cook until the potatoes are tender when pierced with a fork, about 15-20 minutes.
Drain the potatoes and garlic well and return them to the pot.
Add the butter, milk, kosher salt, and black pepper to the potatoes and garlic. Mash with a potato masher or hand mixer until smooth and creamy. Keep warm.
Meanwhile, bring a pot of water to a boil for the green beans.
Add the trimmed green beans to the boiling water and cook for 3-4 minutes until tender-crisp.
Drain the green beans and immediately transfer them to a bowl of ice water to stop the cooking and preserve their bright green color. Drain again.
Toss the green beans with butter, kosher salt, and black pepper.
Serve the roasted chicken thighs alongside the garlic mashed potatoes and steamed green beans.