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1
To make the crust: two to three hours before making the tart place the flours, salt, butter, and shortening into a food processor and pulse until crumbles form that are lima bean size. Slowly add the ice water and pulse until the dough just comes together into a ball. Wrap the dough ball in plastic wrap and refrigerate until ready to bake (at least two hours).
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2
To make the filling: heat the butter and oil in a pan over medium-low heat. When hot, add the onion and thyme and a pinch of salt and pepper and saute gently until the onion caramelizes, about 40 minutes. Set aside.
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3
Bring a pot of salted water to a boil. Parboil the potatoes whole until just about tender, about 15 minutes. Set aside to cool.
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4
Blanch the spinach and kale in boiling water until just barely wilted. Squeeze out any remaining water and toss with the garlic. Set aside.
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5
When you are ready to make the tart, preheat the oven to 375 degrees F. On a lightly floured surface, roll the dough into a circle to fit your tart or pie pan. Place the dough in the pan, pat it in place, and trim off any extra. Prick the bottom of the crust. Cover the crust with aluminum foil, fill it with dried beans and bake for 10 minutes. Remove the foil and beans and bake another 10 minutes. Let cool slightly before adding the fillings.
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6
Assemble the tart: Mix the ricotta, egg yolks, parmesan, and a healthy pinch of salt and pepper. Spread evenly all over the bottom of the crust. Spread the caramelized onions evenly on top of the ricotta mixture. Slice the cooled potatoes into 1/8 to 1/4 inch slices and cover the onions. Sprinkle with another pinch of salt and pepper to season the potatoes. Add the gruyere on top of the potatoes, and lastly add the greens and garlic mixture on top. Bake for 45 minutes, and let rest for five minutes before cutting into wedges. Serve warm.
