In a medium saucepan over medium heat, melt 2 tablespoons of butter. Add 1 finely chopped yellow onion and sauté until softened, about 3-4 minutes. Stir in 2 cloves of minced garlic, cooking until fragrant. Combine ¼ cup of apple cider vinegar, ½ cup of water, 1 cup of ketchup, and the remaining spices. Simmer for about 15 minutes until the sauce thickens, stirring occasionally, then remove from heat.

While the BBQ sauce simmers, take 2 pounds of boneless skinless chicken thighs and pat them dry with paper towels. Rub each thigh generously with olive oil, followed by salt and black pepper to taste.

Preheat your grill to medium-low heat, aiming for about 300°F (150°C). Once hot, place the seasoned chicken thighs skin-side down on the grill. Cook for 6-7 minutes, flipping them every 3-4 minutes.

During the last few minutes of grilling, use a brush to generously baste the juicy chicken thighs with the spicy BBQ sauce. Keep an eye on them to prevent burning.

Transfer the chicken thighs to a plate and let them rest for about 5 minutes before serving.