Bring a pot of water to a boil and boil the cubed red potatoes until tender, 10-15 minutes, until a fork slides in easily. Visual cue: the potato cubes look soft and slightly broken at the edges.

Drain the potatoes, then cool them until no longer steaming, about 10 minutes. Visual cue: you can handle them comfortably and they look matte, not glossy.

Mix the mayonnaise, sour cream, fresh lemon juice, lemon zest, chopped fresh dill, and Dijon mustard until smooth, 1-2 minutes. Visual cue: the dressing turns evenly creamy with flecks of dill.

Combine the cooled red potatoes and finely diced red onion in a bowl. Visual cue: the onion pieces are evenly distributed among the potatoes.

Pour the lemon dill dressing over the potatoes and toss gently until coated, about 1-2 minutes. Visual cue: most potato surfaces look lightly glossy and speckled with dill.

Season with salt and black pepper to taste. Visual cue: the flavor brightens and the dressing tastes balanced.

Refrigerate the potato salad for at least 2 hours to let the flavors meld. Visual cue: it thickens slightly and looks more uniform and chilled throughout.