Cook potatoes in salted boiling water until fork tender. At the same time in a separate pot, hard boil the eggs.
Drain potatoes well and put in large bowl. Place butter directly on potatoes so it can melt. Add Salt, Pepper, Dill, Garlic & Celery Seed to potatoes and let sit.
In the meantime, peel boiled eggs and rinse well. (You don't need to wait for them to cool to peel, just cool enough so that you can handle them safely) Slice up and gently fold into potato & seasonings.
Add your mix ins. I add a cup of pickles (Whatever I have on hand I use but I prefer to use a mix of 3/4 Sweet & 1/4 Dill) and sometimes onion or shallots. Ganna would often put celery in (I think that's because she had it on hand) and I've used mini sweet peppers in the past too. Use your imagination.
In a small bowl whisk together Mustard, Mayo, Sugar & Vinegar. Gently fold "dressing" into potato mix.
Cover & Refrigerate.