In a small bowl, mix together all of the ingredients except for the chicken and the onion.
Add the chicken and the sliced red onion to a large zip-top bag. Pour the marinade in and seal the bag. carefully shake the bag around until everything is coated in the marinade.
Put the chicken in the fridge to marinate overnight or at least 2 hours. (I like to prep this in the morning and then have it ready for dinner!)
Add the chicken and the onions to a 9x9 baking dish or oven safe pan and bake for 40 minutes.
After the 40 minutes, turn your oven to broil and cook for 2-4 more minutes to brown the top. (Keep a very close eye on it!)
Take your chicken out of the oven and let it rest for 10 minutes before serving. You can prepare the Alexia sides while the chicken is resting.