Dice the celery into 1/4-inch pieces and set aside—consistent sizing ensures even texture throughout the salad. Finely chop the dill, including the tender stems, as they add delicate flavor. Slice the green onions, separating the white and light green parts from the darker green tops; set them aside separately since we'll use them at different times. Having everything prepped and ready will make assembly quick once the potatoes are cooked.
Cut the red potatoes into 1-inch chunks and place them in a large pot of cold salted water. Bring to a boil and cook for 15-20 minutes until the potatoes are just fork-tender but still hold their shape—you want them cooked through but not mushy or falling apart. Drain the potatoes in a colander and spread them on a baking sheet or large plate to cool to room temperature, about 15-20 minutes. This cooling step is crucial; warm potatoes will cause the sour cream to break down and create a watery salad.
In a large mixing bowl, whisk together the mayonnaise, sour cream, lemon juice, and Dijon mustard until smooth and well combined. Add the garlic powder, dill, and the white and light green parts of the green onions (reserve the dark green tops for garnish). Taste the dressing and adjust seasonings as needed—I find that a good-quality sour cream like Daisy brand gives you that rich, thick base that makes the whole salad creamy without being heavy, so the seasonings really shine through.
Add the cooled potatoes and diced celery from Step 1 to the dressing and gently fold everything together until the potatoes are evenly coated, being careful not to break them apart. Taste and adjust seasonings with additional salt, pepper, or lemon juice if needed. Transfer to a serving bowl or storage container.
Cover and refrigerate the potato salad for at least 1 hour, allowing the flavors to meld and the salad to chill through. Just before serving, garnish with the reserved dark green tops of the green onions for a fresh pop of color and mild onion flavor. The salad actually tastes even better the next day, so don't hesitate to make it ahead of time for entertaining.