When I made the first Very Thin Tart with sweet potatoes, I tried several versions of topings, but 2 were by far my favourites. I think next time I will make them smaller and serve 2 per person like a duo. The puff pastry is super thin and they came out very light as well. They are best eaten warm although you can make everything ahead and just put the tarts in the oven at the last minute to melt the cheese and re-warm the potato and puff pastry. I used pre-rolled puff pastry sheets, rolled to 1/4 inch thickness and used a 4.5 inch plate to cut around and have all circles the same size. —Maria Teresa Jorge
Pre-heat the oven to 400 degrees F. with rack in the middle. Bring a pot of water to a boil with a dash of salt.
Slice the peeled and washed potatoes with a mandoline to have medium thin slices.
Blanch the potatoe slices for 1 minute, drain and pass them them under cold water to stop cooking and put to dry on a clean tea towel.
Line a baking sheet with parchment paper, lightly smear it with olive oil (preferably garlic infused - see my recipe posted). Using the blanched chips, make a circle overlapping them so you obtain a disk at leats 1/2 an inch smaller then the pastry you cut. Drizzle some olive oil on top, sprinkle with salt, freshly ground pepper and fresh rosemary. Make 6 potato crowns. Bake in the oven for 10 minutes to dry out the potatoes.
Open your puff pastry and if it's thick roll it out so you get to 1/4 inch thickness. You really want these tarts to be extra thin. Roll out the puff pastry between 2 sheets of parchment paper (if you roll out dusting the pastry with flour the puff pastry will end up heavier because it will absorb all the flour).
Remove the top parchment sheet, cut 6 equal circles of 4.5 inch diameter - I used a plate to cut around (or cut in square or rectangle, whichever you prefer).
Important note: always use a very sharp knife to cut the puff pastry and cut down sharply rather then a pulling motion with the knife. You want the pastry to be cut through vertically so it puffs, if you pull it, the layers will stick together and won't puff up.
Cover the puff pastry with the top parchment sheet and chill until you need it.
Grate the scramorza which is dried Mozzarella. The reason for using scamorza is because the water content is very low and won't leave your tart soggy.
Onion Confit: Peel the onions and slice them with a mandoline. In a pot with a tight fitting lid melt the butter over medium low heat. Add the onions and stir for 5 minutes. Cover the pan and let simmer over very low heat another 5 minutes. Add a 1/4 cup of boiling water, stir, cover and let simmer 5 more minutes. Remove the lid and let the remaining water evaporate. Onions should be sweet and soft. Set aside.
Put the puff pastry disks in a baking sheet lined with the parchment paper you used to roll out the dough. With a wide thin spatula, transfer the roasted sweet potato crowns and put them in the middle of the puff pastry. Put the oven rack one up and bake the puff pastry tarts with the potatoes in the oven until the puff pastry has risen and both the potatoes and the pastry are golden.
Divide the onions on top of the potatos on the 6 puff pastries and potato tarts, generously sprinkle the grated scamorza on top, sprinkle with rosemary and freshly ground pepper and put back in the oven until cheese has just melted.
Serve immediately with fresh green salad leaves seasoned with some vinaigrette. If you have guests that don't eat cheese, you can serve it just with the sweet potato and the onions like one of the photos I uploaded.
If making ahead, stop at step 11 and only finish with the cheese when you are ready to serve. The onion confit can be made 2 or 3 days in advance and kept chilled.