In a medium saucepan over medium heat, melt the butter. Add the chopped yellow onion and sauté until softened, about 5 minutes. Stir in the minced garlic and cook for another minute until fragrant.

To the onions and garlic, add apple cider vinegar, water, and ketchup. Stir in the paprika, sriracha, sambal oelek, chili powder, mustard, and Worcestershire sauce. Let this mixture simmer over low heat for 15 minutes, until thickened.

While the BBQ sauce simmers, prep the chicken thighs by patting them dry and drizzling olive oil over them. Season generously with salt and black pepper.

Preheat the grill to medium-low heat (300°F to 325°F) and lightly oil the grill grates.

Place the chicken thighs on the grill skin-side down. Cook on low heat, flipping every 3-4 minutes. Cook until juices run clear and an internal temperature registers 165-170°F, about 20 minutes.

During the last few flips, baste the chicken thighs with the prepared BBQ sauce, ensuring every piece is coated.

Once cooked through, remove the chicken from the grill and let it rest for about 5 minutes. Serve with the reserved BBQ sauce.