Bring a kettle of water to boil. Put a large nonstick pan over medium heat.
Meanwhile, wash the leeks, then finely slice them and add them to the pan with a little of the coconut oil. Stir every couple of minutes.
While leeks are cooking, cut the potatoes into 1/2-inch pieces and put them into a large saucepan. Add enough of the boiling water from the kettle to cover them, and bring water back to a boil, then turn down the heat and simmer for 5 minutes.
Peel and seed the squash, and cut into pieces about the same size as the potatoes. Once the potatoes have had 5 minutes in the boiling water, add the squash to the pan of potatoes, adding more water if necessary, bring back water to a boil and cook vegetables for 3 more minutes. Make sure the potatoes and squash have softened a little, drain them and leave in the colander to steam-dry a bit.
Set aside 3 tablespoons leeks in a bowl. Turn heat up under the leek pan, add a little more oil if necessary, then add potatoes and squash with the remaining leeks and fry, turning every couple of minutes, but not too often — you want to allow each side enough time to build up a bit of a golden crust.
Make dressing while potatoes and squash cook by chopping the herbs and adding them to the reserved 3 tablespoons leeks. Add crème fraîche or yogurt, the lemon juice, and some salt and pepper, and blend well, using a handheld blender. Set aside.