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1
Start caramelizing the onions, since they take the longest.
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2
In a heavy, large skillet or pot, heat half of the butter and olive oil over medium-low heat.
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3
Add the sliced onions and stir.
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4
Sprinkle lightly with salt.
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5
Cook for 40 to 60 minutes, stirring occasionally, until the onions are browning and very stringy.
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6
During cooking, you should hear a constant, very gentle sizzle.
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7
Using too much heat will brown the onions before theyre caramelized, so theyll be missing the sweet taste.
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8
Remove them from the pan and set aside in a bowl when done.
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9
Dont clean the pan because youll need it in a bit.
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10
To make the crust (while the onions caramelize), preheat the oven to 400 degrees F and shred the potato on a box grater or using the grating disc of a food processor.
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11
Place in a colander and sprinkle with salt and pepper.
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12
Wait a few minutes, then use your (clean) hands to squeeze the potatoes into a ball, pressing out as much liquid as possible.
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13
Grease a standard-sized pie plate (glass preferred) generously with softened butter or olive oil.
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14
Sprinkle the grated potato evenly around the pie plate, then press down into a firm layer on the bottom of the pan.
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15
Bake for about 40 minutes, until the bottom of the crust is browning and the top appears dry.
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16
Remove it from the oven and set aside.
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17
Reduce oven temperature to 350 degrees F and place the oven rack just above the middle position.
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18
For the mushrooms, in the pan used to caramelize the onions, heat the butter and olive oil over medium-high heat.
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19
Add sliced mushrooms and stir.
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20
Cook, stirring infrequently, until the mushrooms release their juices and then the juices have evaporated entirely (patience).
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21
They should be just starting to stick to the pan.
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22
Remove from heat and set aside.
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23
For the egg mixture, in a medium bowl, whisk the eggs, half-and-half, and mascarpone until blended.
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24
Add the shredded Fontina and salt and pepper (to your preference, or 1/4 teaspoon each).
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25
To build the tart (the fun part), use a brush to spread the mustard in a thin layer over the cooled crust.
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26
Spread the caramelized onions in an even layer over the crust, then add the cooked mushrooms.
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27
Pour the egg mixture into the pan.
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28
It should work its way into the nooks and crannies, but if not, tilt the pan slightly to help it along.
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29
Arrange the tomato slices gently over the top and sprinkle with salt, pepper and the Parmesan cheese.
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30
Put it into the oven and bake for about 40 minutes, or until the tart is set (it should only jiggle slightly in the middle) and browned on top.
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31
If youre using a different pan size, such as for a double recipe, check the tart earlier and be prepared for a shorter or longer cooking time.
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32
Remove from oven.
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33
Allow to cool before serving so the tart can set up more, giving you nicer-looking pieces.
