Preheat the oven to 425°F (220°C).
In a small bowl, mix 1 teaspoon kosher salt, 1/2 teaspoon black pepper, garlic powder, and dried thyme.
Pat the chicken thighs dry with paper towels. Rub the spice mixture evenly over both sides of the chicken thighs.
Heat olive oil in a large oven-safe skillet over medium-high heat.
Place the chicken thighs skin-side down in the skillet and sear for 5-6 minutes until the skin is golden brown and crispy.
Flip the chicken thighs and transfer the skillet to the preheated oven.
Roast the chicken thighs in the oven for 20-25 minutes, or until the internal temperature reaches 165°F (74°C).
While the chicken is roasting, place the peeled and chopped potatoes along with smashed garlic cloves in a large pot and cover with cold water.
Add 1 teaspoon kosher salt to the pot and bring to a boil over high heat.
Reduce heat to medium and simmer until potatoes are fork-tender, about 15-20 minutes.
Drain the potatoes and garlic well and return them to the pot.
Add 2 tablespoons butter, warmed whole milk, and sour cream to the potatoes.
Mash the potatoes with a potato masher or electric mixer until smooth and creamy.
Season mashed potatoes with 1/2 teaspoon black pepper and additional salt if needed. Remove garlic cloves before mashing if desired.
Remove the chicken from the oven and let it rest for 5 minutes.
Serve the roasted chicken thighs over a bed of creamy mashed potatoes.
Garnish with chopped fresh parsley before serving.