Add half of hte onion and the garlic to the cavity. Greate a paste with the blackening seasoning and olive oil then evenly spread the seasoning over the entire chicken. Place in roasting pan or on a sheet pan and bake. Place the rest of the onion on the roasting pan.
Cook for 20 minutes per pound or until the internal temperature of the chicken breast reaches 165°F or 74°C. The juices will run clear around the puncture site of the meat thermometer. Allow for the chicken to rest 10 minutes before slicing.