Thoroughly dry chicken thighs with paper towels and place them in a large mixing bowl.
Combine salt, black pepper, onion powder, garlic powder, smoked paprika, brown sugar, and red pepper flakes in a small bowl and mix until uniform.
Add olive oil to the chicken thighs and toss to coat evenly. Sprinkle the prepared spice rub over all sides of the chicken thighs, ensuring an even coating.
Preheat a cast iron skillet over medium heat. Place the chicken thighs skin-side down and sear for 4 to 5 minutes until the skin is golden and crisp.
Pour chicken broth and chopped garlic into the base of the slow cooker.
Arrange the seared chicken thighs skin-side up in the slow cooker atop the broth and garlic.
Cover and cook on low for 5 hours, or until the internal temperature of the chicken reaches 185°F.
Carefully remove chicken thighs and serve with desired side dishes.
