Bake the potatoes until the center is tender when you stick a knife in it.
Prepare tart dough. (Use your preferred pie crust. I use 1c flour, 1/3c oil, 1/6 c water, 1/4t salt and blend in food processor, then mold it to the pie dish with the palm of my hand.).
Trim the leeks, and cook 7-10 minutes on medium heat in butter until tender. Add dash of water 3-4 mins into cooking to keep them from burning.
Place all tart ingredients into food processor and blend. Mash the potatoes (skins excluded) and add to the mix.
Cook tart shell for 10 mins at 400.
Pour the tart mix into the shell and bake at 375 for 45-40 minutes or until golden. Gently shake the dish, and if the center giggles loosely, it needs more time.
Note: I made this with evaporated milk instead of cream, and didn't have gruyere so used parmesan and goats cheese. I also didn't have parsley, so added more thyme. I doubled the recipe and used an 11x14 dish. It came out great!