As the first step, preheat the oven to 350°F (180°C). Wash the potatoes well, then dry them.
Prick each potato with a fork in 5–6 places, on all sides.
Rub the potatoes with oil or fat, and coat them with salt on all sides. This is what will make their skins tasty and crisp.
Place the potatoes on a baking sheet lined with parchment paper, and bake for 2 hours. Turn them over roughly halfway through. 20 minutes before the end, raise the temperature to 425°F (220°C). Do not forget, the potatoes need to be baked whole, with their skins on.
Making the egg salad
While the potatoes are baking, cook the eggs. Put them on to cook in lukewarm water, and once the water comes to a boil, cook them over low heat for 8 minutes.
Peel and chop the eggs. Peel them under cold running water, dry them, and mash them in a bowl with a fork. They do not need to be mashed very finely.
Peel the onion and chop it very finely.
Add the remaining ingredients to the eggs: the chopped onion, mayonnaise, salt, and pepper. Mix well.
Serving the potatoes
Take the potatoes out of the oven. With a sharp knife, cut each one lengthwise quite deeply, then cut across a little as well. Open up the potato with a knife and fork, and lightly scrape the inside with the fork. Be careful, because at this stage the potato is very hot. Touch the baking sheet only with oven mitts, and the potato only with utensils.
Spoon 2–3 tablespoons of egg salad mixture into each potato, to taste. Serve immediately.You can also flavor it with spinach or grated cheese, for example, to taste.
Notes
Plan on about 3½ oz (100 g), or 3 tablespoons, of egg salad per potato.