Cut the potatoes into 1-inch cubes for even cooking throughout. Bring a large pot of water to a boil and add 1.5 teaspoons of salt. Add the potato cubes and boil for 15-20 minutes until they're fork-tender but still hold their shape—you want them cooked through but not falling apart. Drain the potatoes in a colander and spread them on a sheet pan to cool for about 30 minutes. This cooling step is crucial because it allows the potatoes to firm up slightly, which prevents them from becoming mushy when mixed with the dressing.

While the potatoes cool, combine the Greek yogurt and Dijon mustard in a large mixing bowl, whisking them together until smooth and well incorporated. Add the garlic powder, black pepper, and most of the fresh dill (reserve a small handful for garnish), stirring until everything is evenly distributed. Taste the dressing and adjust the seasoning if needed—I like to add a pinch more salt here to ensure the flavors are well-rounded since the potatoes will absorb some of the dressing's seasonings.

Once the potatoes have cooled completely, add them to the dressing along with the finely diced red onion. Fold everything together gently but thoroughly, making sure each potato piece is coated with the creamy dressing. The red onion will add a fresh crunch and subtle bite that balances the richness of the Greek yogurt. Taste and adjust seasonings one final time, then transfer to a serving dish and garnish with the reserved fresh dill for a bright, fresh finish.