Peel and cut the potatoes into 1/2-inch cubes.
Place them in a large pot filled with water.
Bring to a boil.
Reduce the heat to medium-low.
Cover the pot and cook until the potatoes are tender but firm, about 20-25 minutes.
Drain the potatoes and run cold water over them to cool them quickly.
Peel the hard-boiled eggs and chop them roughly.
Add the chopped eggs to a large mixing bowl with the cooled potatoes.
Add the chopped onion, green pepper, and celery to the bowl with the potatoes and eggs.
Sprinkle with celery seeds, seasoning salt, black pepper, and red pepper flakes.
In a separate bowl, mix together the mayonnaise, sweet relish, and mustard.
Pour the mayonnaise mixture over the potato mixture.
Stir gently to combine until everything is evenly coated.