Preheat the oven to 350°F. Line a baking sheet or use a shallow baking dish.
Pat the 1 pound chicken breast dry. Brush all sides with 1 tablespoon vegetable oil, then sprinkle 1/2 teaspoon garlic powder and 1/4 teaspoon celery salt evenly over the chicken.
Place the chicken on the prepared baking sheet and bake for 25 minutes, or until cooked through (internal temperature 165°F).
Remove the chicken from the oven and let it cool until it is easy to handle (about 10 minutes). Chop the chicken into small, bite-sized pieces.
In a large mixing bowl combine the chopped chicken, 1/2 cup finely diced celery, 1/4 cup finely diced onion, and 3 hard-boiled eggs, chopped.
Add 1 tablespoon sweet relish, 1/2 cup mayonnaise, and 1 tablespoon dried parsley to the bowl. Mix until everything is evenly combined.
Taste the salad and season with salt and pepper to taste.
Cover and chill in the refrigerator for at least 1 hour to allow the flavors to blend. Serve chilled.