Bring a pot of water to a boil over high heat, then add the red potatoes and boil until tender, about 15-20 minutes, with the potatoes showing slight give when pierced. Drain and cool until just warm.

Spread the boiled potatoes out and let them cool to room temperature, about 10-15 minutes, so the dressing doesn’t thin out. Visual cue: potatoes look dry on the surface and hold their shape.

In a mixing bowl, whisk together mayonnaise, sour cream, fresh lemon juice, lemon zest, fresh dill, and Dijon mustard until smooth, about 1-2 minutes. Visual cue: dressing turns evenly yellow-green and clings lightly to the whisk.

Add the cooled potatoes and the red onion to a bowl and toss gently to distribute the onion. Visual cue: onion pieces are visible throughout the potatoes.

Pour the lemon dill dressing over the potatoes and toss gently until coated, about 2 minutes. Visual cue: the potatoes look glossy with an even creamy layer.

Season with salt and pepper to taste and toss again briefly to ensure even flavor, about 30-60 seconds. Visual cue: seasoning makes the lemon flavor pop without dry spots.

Cover and refrigerate for at least 2 hours before serving. Visual cue: the salad looks thicker and the dressing clings more tightly to the potatoes.