Prepare the onions: Heat a large skillet over medium heat and add the olive oil. Once hot, add the thinly sliced onions and cook for 5-7 minutes, stirring occasionally, until they become soft and caramelized.

Cook the chicken thighs: In the same skillet, add the boneless, skinless chicken thighs. Season with a little salt and pepper if desired. Cook on each side for about 5-6 minutes, until they are golden brown and fully cooked through.

Add the BBQ sauce: Once the chicken thighs are cooked, reduce the heat to low and pour the lower-carb BBQ sauce over the chicken. Make sure each thigh is coated evenly. Let the sauce simmer with the chicken for 2-3 minutes to allow the flavors to blend.

Top with cheese and onions: Sprinkle the shredded cheese evenly over the chicken thighs, then top with the caramelized onions. Cover the skillet with a lid and let it cook for another 1-2 minutes, or until the cheese has melted.

Serve: Serve the BBQ chicken thighs hot, and enjoy them on their own or with your favorite low-carb sides such as a fresh salad or roasted vegetables.