Place the potato cubes in a large pot and cover with cold water by 2 inches. Add the tablespoon of salt and bring to a boil over high heat. Reduce to a simmer and cook until just tender when pierced with a knife, about 8-10 minutes. Do not overcook - you want them holding their shape.

While the potatoes simmer, set a steamer basket over a separate pot with 1 inch of simmering water. Add cauliflower florets, cover, and steam until crisp-tender, about 5-6 minutes. The cauliflower should yield slightly to a fork but still have bite. Transfer to a colander and rinse briefly with cold water to stop cooking.

When potatoes are tender, drain in a colander and spread on a baking sheet in a single layer. Let cool to room temperature, about 15 minutes. This prevents them from turning mushy in the dressing.

In a large mixing bowl, whisk together mayonnaise, Greek yogurt, Dijon mustard, apple cider vinegar, and celery seed until smooth and well combined. Taste and season with salt and black pepper - the dressing should be tangy and well-seasoned since it will mellow once mixed.

Add the cooled potatoes, steamed cauliflower, diced celery, red onion, chopped dill, and hard-boiled eggs to the bowl with the dressing. Fold gently with a rubber spatula until everything is evenly coated. Take care not to break up the potatoes or eggs too much.

Cover and refrigerate for at least 30 minutes to let flavors meld. Taste again before serving and adjust seasoning if needed. Garnish with additional fresh dill and a sprinkle of paprika if desired.