Prepare the veggies: Peel the potatoes, cut into chunks, place in a large pot, cover with water, season with 1 teaspoon of salt, bring to a boil and cook until fork tender. Drain and let cool. Boil the eggs for 10 minutes, peel and set aside. Cook the green beans until ready. Set aside.
Thinly slice the onion, cut in half cherry tomatoes and hard boiled eggs. Set aside.
Place all vegetables in a large bowl and set aside.
Make the dressing: in a medium size bowl whisk mustard, vinegar, salt and pepper. While whisking slowly pour olive oil. Set aside the dressing.
Mix all the vegetables with half of the dressing.
Lay the lettuce leaves onto a large plate. Transfer the salad to the plate.
Cook the tuna: Season the tuna with salt and pepper from both sides. Heat olive oil over medium heat and cook the tuna 2 minutes from each side. Slice and place on top of the salad.
Cut boiled eggs in half and add to the salad. Season with salt and pepper. Drizzle the remaining dressing.
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Notes
Different Protein: Instead of tuna, you can use salmon, shrimps, shredded chicken, or even a vegan protein such as tofu, chickpeas, or seitan.
Vegan: Skip the boiled egg and use a plan-based protein of your choice to turn this salad into a vegan or vegetarian Nicoise.
Vinegar: You can use white wine or red wine vinegar in the dressing, depending on preference.
Vegetables: Add more vegetables to your liking (red pepper, broccoli, carrots, etc). Don't overcook the vegetables. They'll get too soft and lose their shape and flavor if they get overcooked.
Use fresh and high quality ingredients for the best result and flavor.
Egg: If you like a runny egg yolk, soft boil the eggs by reducing the cooking time to 7-8 minutes.
Presentation: Assemble the plate in a colorful and nice presentation if you have guests. Serve at room temperature for the best flavor.
Tuna: Instead of fresh tuna fillet you also can use canned tuna.