As you can see by the title, this is not Grandma's recipe. But a few years ago I found one that was similar, and I've tweeked it a little each time I make it. One thing that Grandma does differently: she adds the potatoes while they're still somewhat warm. I think that's because they absorb the flavors a little more. But you have to be careful if you do it that way, because it's even harder to stir them without mashing them.
not Grandma’s Potato Salad
5 pounds red potatoes
1 large onion, chopped
2 1/2 cups mayo
1/4 cup prepared yellow mustard
6 hard-cooked eggs, diced
salt and pepper to taste
Place the potatoes in a large pot and fill with enough water to cover. Bring to a boil, then reduce heat to medium and cook until tender enough to pierce with a fork, about 15 minutes. Drain and cool slightly. Remove the peels and cut into cubes. Set aside to cool completely.
In a large bowl, mix together the onion, mayo, and mustard. Stir in the hard-cooked eggs and cooled potatoes. Try not to mash the potatoes very much. Season with salt and pepper.