Heat the oil in a heavy large frying pan set over medium heat.
Add the pancetta and saute until brown and crisp, about 8
minutes. Remove the pancetta from the pan and set aside to
cool.
Add the onions to the pan and cook until golden brown and
lightly caramelized, 10 minutes. Add the garlic and 1/2
teaspoon of the salt and cook 2 minutes more. Set aside to
cool slightly. In a large bowl, whisk together the gruyere,
cream, Parmesan, lemon zest, eggs and the remaining 1/4
teaspoon salt.
Meanwhile, bring a large pot of salted water to a boil over high
heat. Add the pasta and cook until just tender but still firm to
the bite, stirring occasionally, 8 to 10 minutes. Drain, reserving
1 cup of the pasta water. Add the pasta back to the pan along
with the onions, cream mixture, 1/4 cup of the reserved pasta
water and the pancetta. Toss over low heat until the sauce is
thick and coats the pasta, adding pasta water as needed,
about 2 minutes (do not boil)
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