In medium bowl, whisk together all sauce ingredients until well combined. Set aside.
Preheat oven to 375° Fahrenheit. Lightly season chicken with salt and pepper. Set aside.
Place onion slices in bottom of 9x13 baking dish. Set aside.
Add avocado oil to large skillet and warm over medium heat. Once oil is hot, place chicken in skillet (working in batches as needed) and cook 3 to 4 minutes, until browned. Flip chicken and cook another 3 to 4 minutes until browned on other side. Repeat with any remaining chicken.
Transfer browned chicken to baking dish on top of onions, placing chicken in one even layer.
Pour sauce mixture over chicken, coating thoroughly. Cover baking dish with aluminum foil and place in preheated oven. Bake, covered, 25 minutes.
After 25 minutes, remove foil from baking dish. Return uncovered dish to oven and bake another 20 to 25 minutes or until chicken is fully cooked through. Divide chicken and onions evenly onto plates and serve warm with desired sides.
Notes
Chicken: Any type of chicken will work here - boneless or bone-in; thighs, drumsticks, breasts. I used bone-in thighs and drumsticks.