Bring a Dutch oven of water to a boil over high heat, then add the cubed red potatoes and cook until tender, about 10–15 minutes. You should be able to pierce a cube easily with a fork without resistance.

Drain the potatoes and spread them on a sheet pan to cool to room temperature, about 15–20 minutes. They should feel warm-not-hot before dressing so the mayo doesn’t loosen.

Whisk mayonnaise and sour cream together in a bowl until smooth, about 30–60 seconds. The mixture should look creamy and evenly combined.

Whisk in fresh lemon juice, lemon zest, chopped dill, and Dijon mustard until glossy and thick, about 45–60 seconds. The dressing should be bright yellow-green flecked with dill.

Combine the cooled potatoes and finely diced red onion in a bowl and toss gently until evenly distributed. The onion pieces should be visible throughout the potato cubes.

Pour the lemon-dill dressing over the potatoes and toss gently until the cubes are coated, about 1–2 minutes. Everything should look lightly sauced, not swimming.

Season with salt and pepper to taste and toss again for even flavor. Taste one potato to confirm it’s bright and balanced.

Cover and refrigerate the potato salad for at least 2 hours before serving. It should look thicker as the dressing chills and clings to the potatoes.