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1
In a large frying pan over high heat, cook onions in 3 tablespoons olive oil with sugar and 2 teaspoons salt, stirring and turning often, until onions begin to release their juices and turn golden, 10 to 13 minutes.
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2
Reduce heat to medium and cook, stirring occasionally, until onions turn a caramel color and become quite sweet, 35 to 40 minutes more.
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3
If onions begin to stick to the pan or char during cooking, reduce heat.
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4
Meanwhile, in a small frying pan (not nonstick) over medium-low heat, toast walnuts or cashews, stirring frequently, until fragrant and golden, about 10 minutes.
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5
Remove from heat and cool slightly.
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6
Pour nuts into a zip-lock plastic bag and lightly crush with a rolling pin.
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7
Set aside.
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8
When onions are nearly done, cook pasta in boiling salted water until tender to the bite, 9 to 12 minutes or according to package instructions.
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9
Drain pasta, reserving about 1/2 cup cooking water.
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10
Toss caramelized onions with pasta, walnuts, ricotta salata, parsley, reserved cooking water, lemon juice, pepper, and remaining 2 tablespoons olive oil.
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11
Season to taste with salt & crushed red pepper flakes, if using
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12
Serve hot.
