Trim thighs of any excess fat and pat dry. Place chicken in a shallow pan and drizzle with 2 tbsp olive oil. Mix garlic, onion powder and salt and sprinkle all over. To ensure the chicken gets well coated, use your hands to massage seasoning into chicken.
Let the chicken sit for 20 minutes in the dry rub, and begin making the BBQ sauce. Sauté onion in a small sauce pan on medium heat, till tender
Add ketchup, Worcestershire sauce, steak sauce, water, apple cider vinegar and brown sugar. Stir well. Raise to medium-high heat and bring to a gentle boil, then reduce to low. Cover and cook for about 20 minutes.
Place the thighs on the hot grill and cook, covered, for about 7 minutes per side. Once the bbq boneless chicken thighs have nice grill marks and the juices run clear when pierced, liberally baste the thighs with the barbecue sauce.
Using about ½-1 cup of the sauce, baste one side, turn and cook for about 2 minutes and baste the other side. If using a meat thermometer, the internal temperature of the thighs should read 160 degrees Fahrenheit.