Preheat oven Preheat oven to 425F (220C). Let it fully preheat for at least 10 minutes after the signal - ovens often read ready before they have fully stabilized at temperature.
Pound to even thickness Place chicken breasts between two sheets of plastic wrap or in a zip-top bag. Using a meat mallet or rolling pin, pound the thickest part down until the entire breast is an even 3/4 inch thick. This is the single most important step for juicy baked chicken breast - uneven thickness means uneven cooking.
Dry and oil the chicken Pat each chicken breast completely dry with paper towels on both sides. Drizzle avocado oil over each piece and rub to coat evenly. Surface moisture creates steam and prevents browning - patting dry takes 30 seconds and makes a visible difference.
Season generously Mix all spice blend ingredients in a small bowl. Sprinkle generously over both sides of each chicken breast, pressing lightly so the seasoning adheres. Do not be shy with the seasoning - chicken breast has no skin and needs the spice blend to carry the flavor.
Bake Place seasoned chicken breasts on the rimmed sheet pan with space between each piece. Bake at 425F for 18 to 22 minutes depending on size. 6 oz breasts take about 18 minutes, 7 to 8 oz take about 20 minutes, and anything over 9 oz takes 22 minutes or more.
Check temperature At 18 minutes, check the internal temperature at the thickest point with an instant-read thermometer. Pull the juicy baked chicken breast from the oven at 158 to 160F - not 165F. Carryover cooking will bring it to 165F during resting. According to chicken is safe at 165F internal temperature.
Rest Remove from the oven and rest on the pan for 5 full minutes. Do not cut into it yet. Resting allows the juices that have been pushed toward the center during cooking to redistribute back through the meat.
Serve Slice against the grain for the most tender texture - cutting across the muscle fibers shortens them, which makes each bite easier to chew. Spoon any pan drippings over the chicken or over whatever side dishes you are serving alongside.