Wash the baby potatoes well and place them in a large saucepan with salted water. Boil until fork-tender but not overly soft, usually around 12 to 15 minutes. Drain them and let them cool slightly before cutting into bite-sized pieces.
Place the eggs in another saucepan and cover with water. Bring them to a boil, then simmer for about 10 minutes until hard-boiled. Transfer the eggs into cold water immediately so they peel easily and stay tender.
In a small bowl, whisk together olive oil, harissa paste, lemon juice, preserved lemon, honey, salt, and black pepper. Taste the dressing and adjust the spice level if needed. The dressing should taste smoky, tangy, and slightly spicy.
Add the cooked potatoes, sliced onion, tomatoes, olives, and parsley into a large bowl. Gently fold in the tuna so the chunks stay intact and do not become mushy. Pour most of the dressing over the mixture and toss carefully.
Peel the eggs and cut them into halves or quarters. Arrange them on top of the salad for a beautiful presentation. Drizzle the remaining harissa dressing over everything before serving.
You can enjoy the salad immediately while slightly warm or chill it for about 30 minutes. Chilling helps the potatoes absorb more flavor from the dressing. Serve with crusty bread or warm pita for a complete meal.