Step 1

In a large deep frying pan, cook the bacon over moderate heat until crisp. Remove the bacon with a slotted spoon. Pour off all but 3 tablespoons of the bacon fat or, if you don't have 3 tablespoons, add enough olive oil to make up the amount. Put the pan over moderately low heat. Add the onions and 1/4 teaspoon of the salt. Cook, covered, for 5 minutes.

Step 2

Uncover the pan and continue cooking, stirring occasionally, until the onions are golden, 10 to 15 minutes. Stir in the scallion bulbs and the garlic; cook 2 minutes longer, stirring occasionally. Add the broth and bring to a simmer.

Step 3

Meanwhile, in a large pot of boiling, salted water, cook the spaghettini until almost done, about 9 minutes. Drain the spaghettini and add it to the simmering broth. Cook until the pasta is just done, about 3 minutes. Stir in the Parmesan and the remaining 1/4 teaspoon salt. Toss with the bacon and scallion greens.