Boil the red potatoes in a Dutch oven over high heat until tender, 10-20 minutes, until a knife slides in easily. Transfer to a sheet pan and drain any excess water.

Cool the potatoes to room temperature for 15-20 minutes, so the dressing won’t break or thin out. The surface should look dry and matte, not steaming.

In a bowl, whisk mayonnaise, sour cream, fresh lemon juice, lemon zest, fresh dill, and Dijon mustard until smooth, 1-2 minutes. The dressing should look glossy and evenly speckled with dill.

Combine cooled potatoes and red onion in a bowl. Toss gently so the onion is distributed without crushing the cubes.

Pour the lemon dill dressing over the potatoes and toss gently until every piece is coated. The salad should look bright and lightly creamy, with dill visible on the surfaces.

Season with salt and pepper to taste and toss once more to combine. Add gradually, then stop when flavor is balanced and the lemon tastes present but not sharp.

Refrigerate the potato salad for at least 2 hours, covered, so flavors meld. Serve cold; the potatoes should hold shape and the dressing should cling to them.