I'm addicted to markets, where I find seasonal vegetables, buy a bit of this, a bit of that and then cook from what I have. Autumn is the season for Porcini mushrooms and squash so I made this gratin with potatoes, layering the vegetables like a lasagna. The recipe looks very long but it's very easy to do. You only have to cook all the ingredients first and then layer them with the bechamel sauce and cook the gratin in the oven. You can do-ahead the vegetables and the bechamel and layer and cook the gratin when you are ready to serve. —Maria Teresa Jorge
In a medium saucepan, melt the butter until it starts to get a little blond. Add the flour all at once and stir continuously over low heat until well blended.
Slowly add the warm milk, stirring constantly (I actually prefer to use a whisk, I think it helps blending better). When it's all incorporated, continue cooking for 15 minutes over low heat. You need to cook the bechamel sauce for this amount of time to cook the flour and minimize the taste of raw flour. Stir in the parmesan cheese.
Season with salt, freshly ground white pepper, a nice dash of freshly grated nutmeg and let cool. The bechamel should remain soft, not thick.
Beat the egg white to soft peaks and lightly fold it in the cool bechamel sauce. Set aside. If doing the bechamel sauce ahead, only beat the egg whites and add them when you are ready to make the gratin.
Vegetables for filling
For roasting the squash: Pre-heat the oven to 300ºF with rack in the middle.
Line a baking tray with parchment paper.
Cut off the stem ends of the squash, then cut in 4 lengthwise. Discard the seeds, then cut squash into 1/2-inch-thick slices and peel off the skin. Transfer slices to a bowl, season with 1 tablespoon of olive oil, a pinch of salt, freshly ground black pepper and toss to coat.
Arrange in 1 layer in the lined baking tray and roast slowly until golden, about 15 minutes. Turn squash over with a spatula and cook for a further 15 minutes.
For the Porcini mushrooms: Clean the fresh porcini mushrooms with a damp cloth and if needed a brush to get rid of any earth. Gently twist the stems from the caps. Cut the earthy part of the stem, peel the outside and then slice the stems. Cut the caps into 1/4-inch-thick slices.
Peel and remove the inside green part of the garlics. Mince.
In a heavy skillet over high heat, heat 4 tablespoons of extra Virgin Olive Oil. Add the mushrooms and spread them in a layer. Leave to cook without stirring until they are golden brown, then add the garlic and calamint or parsley and stir. Let cook for a further 7 minutes, always over high heat so they don't let out liquid. If you use really fresh Porcinis, they won't let out water. Season with salt and freshly ground black pepper and set aside.
For the potatoes: Bring a large pot of water to boil, season with salt.
Wash and peel the potatoes. Using a mandolin, slice the potatoes thinly (no need to wash the potatoes. Cook 1/3 of the potato slices at a time in the boilng water and as soon as the water returns to a boil, cook for a further 2 minutes, Remove potato slices from pot and spread them on a clean tea towel. Continue cooking the remaining potatoes until you are finished, always putting them on clean tea towels to dry.
Preparing the gratin: Spread 2 tablespoons of Extra Virgin Olive Oil on the bottom of a 7" by 11" gratin dish. Divide the potatoes in 5 equal parts, the bechamel in 5 equal parts, the Porcini in 2 equal parts and the squash in 2 equal parts.
Layer the gratin as follows: At the bottom start with a layer of blanched potatoe slices without overlaying them. Then follow in this order over the potatoe layer - bechamel, squash (1/2), potaoes, bechamel, porcini (1/2), potatoes, bechamel, squash (1/2), potatoes, bechamel, porcini (1/2), potatoes and a final layer of bechamel.
Smoothen the top bechamel layer with a spatula, sprinkle with the 3 tablespoons of grated Parmesan cheese and cook in a pre-heated oven at 350ºF for 20 minutes or until the top is golden colour. (all the ingredients are cooked previously so you only want to rewarm and blend the ingredients.
Serve with a green salad or as a side dish with roast meat.
Note: Calamint is a herb from the mint family and in Italy it grows wildly and is the best herb to use with Porcini mushrooms. The italian name is Nepitella. If you cannot find Calamint, use parsley as mint will overpower the porcini's taste.