Pat dry the chicken thighs and season them generously with salt and pepper on both sides.
In a small bowl, combine cumin, garlic powder, and onion powder. Distribute the seasoning on both sides of the chicken thighs.
Sear:
Place a large cast-iron skillet over medium-high heat and warm up the pan. Once it's hot, add the olive oil. Place the chicken thighs skin-side down and sear until the skin is golden and crispy, about 7 minutes, without moving them. This is a good time to use a splatter guard (screen) if you have one. If you use a lid, don't cover the pan completely, you don't want to build steam inside.
Using kitchen tongs, flip the chicken thighs over and place a rosemary sprig under the chicken. This cooks the rosemary flavor right in! You can also place it on the sides.
Add the butter, whole garlic cloves, and continue cooking the chicken thighs on the skillet, over medium-high heat, for 12 minutes, until the chicken has reached an internal temperature of 175F.
Turn off the heat. Spoon the garlic butter sauce over the thighs to coat and serve.
To finish it in the oven instead:
While you sear the thighs, preheat the oven to 400F.
Flip the thighs over, add the rosemary, garlic, and butter into the skillet. Then, transfer the skillet into the preheated oven and finish cooking the chicken thighs for 18 to 20 minutes until their internal temperature has reached 175F.