In a medium saucepan over medium heat, melt the butter. Add the chopped onion and sauté until translucent, about 5 minutes. Stir in the minced garlic and cook for another minute until fragrant. Add apple cider vinegar, water, ketchup, paprika, sriracha, chili powder, mustard, and Worcestershire sauce. Simmer for about 15 minutes, stirring occasionally, until the sauce thickens.

While the BBQ sauce is simmering, pat the chicken thighs dry with paper towels. Drizzle olive oil over the chicken, and season with salt and black pepper. Rub the oil and spices into the chicken, then let the thighs rest at room temperature.

Preheat the grill to medium-low heat, aiming for 300-350°F, and ensure the grill grates are clean and lightly oiled.

Place the chicken thighs smooth side down on the grill and cook for about 20 minutes, turning every 3-4 minutes for even cooking. Look for an internal temperature of 165-170°F to confirm doneness.

During the last 5 minutes of grilling, brush the BBQ sauce over the chicken. Continue cooking until the sauce caramelizes slightly.

Remove the chicken from the grill and let it rest for a few minutes before serving. Drizzle with any remaining sauce and serve hot.