In a very large bowl (read: HUGE) or utilizing a big roasting pan, mix together the onions, radishes, and eggs.
When potatoes are done, cut while still warm (although it says up above to peel, it's up to you whether or not you want to leave the skins on) and fold together in the large bowl or roasting pan with the onions, radishes, and eggs.
Then add mayonnaise, mustard, garlic powder, celery seed, salt, pepper, and celery seed- it makes it taste better to combine while warm.
Mix well, transfer to serving dish and sprinkle the top of the potato salad with additional celery seed for garnish.
Cover and chill for at least 2 hours before serving.