1. In a medium-sized mixing bowl, combine the chopped hard-boiled eggs and flaked tuna.
2. Add the diced celery and onion to the bowl, mixing these in with the eggs and tuna.
3. In a small bowl, whisk together the mayonnaise and mustard until smooth and well combined.
4. Pour the creamy dressing over the egg and tuna mixture. Use a spatula or spoon to gently fold everything together, ensuring the ingredients are evenly coated with the dressing.
5. Season the salad with salt and pepper to taste. Mix gently once more to distribute the seasonings.
6. Cover the bowl with a lid or plastic wrap and refrigerate for at least an hour to allow the flavors to meld together.
7. Before serving, give the salad a final stir and adjust seasoning if necessary. (Optional: Smoked paprika to garnish)

8. Serve as desired, whether heaped onto sandwich bread or as a topping for a crisp salad. Enjoy your Amish Creamy Tuna Egg Salad as a reflection of humble, satisfying comfort food.