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1
Peel the onions, then chop them into quarters.
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2
Slice the onions so that the pieces form crescent-shaped strips about 3 long.
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3
Melt 2 tablespoons of butter in a large frying pan over medium heat.
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4
Once butter has melted, add the onions and salt and toss well.
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5
Cook onions over medium heat for about 30 minutes, stirring often to caramelize all the onions.
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6
When the onions have been cooking for about 10 minutes, bring a large pot of salted water to a boil.
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7
Add the spaghetti and cook according to package instructions for al dente.
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8
As the spaghetti cooks and the onions are mostly caramelized, move the onions to one side of the frying pan and add the last tablespoon of butter to the cleared side of the pan.
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9
Add the chopped mushrooms to the pan and saute for 2-3 minutes.
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10
Mix the mushrooms into the onions and add pepper and balsamic vinegar.
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11
Add the cream and reduce heat to low, stirring well to incorporate all the cream.
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12
Drain the pasta into a colander, reserving about 1 cup of the pasta water.
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13
Return the pasta to the pot and pour the onion mixture into the pot on top of the pasta.
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14
Add about 1/2 of the reserved pasta water and mix well until thoroughly combined, adding more pasta water if necessary.
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15
Adjust seasoning to taste.
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16
Plate the spaghetti and add a few Gorgonzola crumbles as a garnish.
