In a large pot, bring 4 quarts water to a boil. Add the pasta and 1 tablespoon salt, then cook, stirring occasionally, for 5 minutes. Stir in the kale and cook, stirring occasionally, until the pasta is al dente. Reserve 1 cup of the cooking water, then drain.
In the same pot over medium, combine the oil, pancetta and garlic; cook, stirring, until the pancetta is browned and crisped, 5 to 7 minutes. Off heat, remove and discard the garlic. Using a slotted spoon, transfer the pancetta to a paper towel-lined plate.
To the fat remaining in the pot, add the pepper flakes. Cook over medium, stirring, until fragrant, about 30 seconds. Add the pasta-kale mixture and toss. Add the Parmesan, then stir in ½ cup of the reserved pasta water. The pasta should be lightly sauced; add more reserved water 1 tablespoon at a time as needed to thin. Off heat, stir in the lemon juice. Taste and season with salt and black pepper. Serve sprinkled with the pancetta and additional cheese and drizzled with additional oil.
