Start by preheating the oven to 400°F and lining a rimmed baking sheet with foil {this is for easier clean-up; this step is optional}. Make sure that the oven racks are in the bottom third of the oven.
Put about ⅔ of onion {about 1 cup} in a pile in the center of the baking sheet. Set aside the remaining onion (to go inside the cavity of the chicken).
Open your chicken from the package and take out the insides; discard. Don’t bother washing the chicken. Pat the chicken really dry.
Rub the very dry chicken with 2 teaspoons of oil, coating the entire chicken and under the skin.
Then rub with salt and the spices, if using.
Finish up the chicken by filling the chicken with lemon and some chopped onion.
Now transfer your well-seasoned and filled chicken to the baking sheet. Put in on top of the onion, breast side up. Set the timer for 60 minutes.
Meanwhile, chop your brussels sprouts and toss with remaining 4 teaspoons olive oil and ¼ teaspoon salt. Put on a baking sheet. Add to the oven about 30 minutes into cooking the chicken.
Roast everything until the chicken is done (test with a meat thermometer until it registers as 165°F). Mine usually takes about 75 minutes.