How to Make It
In a large frying pan, cook the bacon until crisp. Remove the bacon with a slotted spoon and drain on paper towels. Pour off all but 2 tablespoons of the bacon fat. Put the pan over moderate heat. Add the onions, the thyme, the red-pepper flakes, and 1/4 teaspoon of the salt. Cook, stirring occasionally, until the onions are brown, about 10 minutes. Remove from the heat.
In a large pot of boiling, salted water, cook the linguine until just done, about 12 minutes. Reserve 1 cup of the pasta water. Drain the linguine and toss with the bacon, onions, 3/4 cup of the reserved pasta water, the Parmesan, parsley, the remaining 3/4 teaspoon salt, and the pepper. If the sauce seems too thick, add more of the pasta water. Serve with additional Parmesan cheese.
Variation: Linguine with Leeks, Pancetta, and Parmesan Substitute 1/2 pound of pancetta for the bacon, and 1 1/2 cups of sliced leeks, white and light-green parts only (from about 3 medium leeks), for the onion.