1. Choose potatoes of roughly equal size. Cook them in a large pot filled with boiling water.
2. Once cooked, drain them and peel them (be careful, it's hot!)
3. Mash them using a potato ricer (choose the finest grid).
4. Meanwhile, heat the milk in a small saucepan.
5. Once hot, add it to the mashed potatoes and mix everything with a whisk.
6. Incorporate the diced butter while whisking.
7. Finish by seasoning with nutmeg, a few turns of the pepper mill, and fleur de sel.
8. Place the mashed potatoes in ramekins. Slightly hollow out the center (using the back of a large spoon) and crack an egg. Season with salt and pepper.
9. Put in a preheated oven at 200 °C for about 15 minutes. The egg white should be cooked.
10. Mix some olive oil, mustard, salt, and pepper and pour over the salad. Mix.
11. Place a few lettuce leaves on the plates, place the ramekins, and serve.