Put the potatoes in a medium saucepan, cover with water by an inch or two, add a large pinch of salt, and bring to a boil. Reduce the heat to medium, partially cover, and cook until the potatoes are tender , about 20 minutes. Test for doneness by spearing a potato with a thin metal skewer. It should penetrate easily into the center of the potato and then slide right out. If the skewer lifts the potato out of the pot when you withdraw it, continue cooking a little longer. Drain the potatoes and let them cool. You can drain them on a cooling rack set over or in your sink, which will avoid squashing the tender potatoes (as often happens with a colander) and also lets the potatoes cool quickly.

When the potatoes are at room temperature, cut them into 3/4-inch chunks and put them in a mixing bowl. Add the celery, radishes, scallions, and dill and fold gently to distribute; set aside.

In a small bowl, whisk the cream until frothy but not at all stiffened. Whisk in the mayonnaise and mustard. Add the lemon juice, zest, 1/2 teaspoon salt, and pepper to taste. Pour the dressing over the salad and fold it in with a rubber spatula. Taste for seasoning. Serve, or cover and chill for up to a day.