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1
Cut the butter for the tart dough into cubes.
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2
In a food processor, quickly pulse the flours and butter together until small clumps form (if mixing by hand in a bowl, use a fork).
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3
Add the cottage cheese and salt and pulse until combined.
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4
Finally, add the water and pulse until the dough comes together.
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5
Turn the dough out and knead until smooth.
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6
Wrap in plastic and let rest in the fridge for 20 minutes.
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7
Place the dough on a lightly floured work surface, knead it a bit, and roll it out into a disk about 1/8 inch thick.
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8
If needed, add more flour to the surface so that the dough doesnt stick.
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9
Line a 9-inch springform or pie pan with the dough.
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10
Be certain to press the dough into the corners of the pan.
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11
Cut away any extra dough that may be hanging over the sides.
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12
Cover the shell with plastic wrap and let it rest in the fridge for about 30 minutes.
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13
Preheat the oven to 400F.
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14
Take the pie shell out of the fridge and remove the plastic wrap.
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15
Prick the bottom with a fork, cover the top edges with aluminum foil, and prebake the shell in the middle of the oven for about 15 minutes or until the bottom is baked through and begins to brown.
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16
Take the pie shell out of the oven and remove the aluminum foil.
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17
Thinly slice the onion for the filling.
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18
Heat a frying pan with the butter and fry the onions until soft without letting them brown.
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19
Add the sugar and vinegar and let the liquid boil down.
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20
Season with salt and pepper.
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21
Chop the mushrooms and sage and stir them together with the eggs, milk, and cream in a bowl.
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22
Season with salt and black pepper.
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23
Pour the egg custard into the pie shell, distribute the cooked onions over the custard, and top with Parmesan cheese.
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24
Place the tart back into the oven and bake in the middle of the oven until the filling has set, about 20 minutes.
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25
If desired, and if youve used a broiler-safe pan, dust the dish with Parmigiano-Reggiano, set the skillet 4 to 6 inches from a heated broiler and broil until lightly brown, 1 to 2 minutes.
