Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs; drain.
Meanwhile, heat the olive oil in a large skillet over medium heat. Add the pancetta and cook, stirring occasionally, until crisp, about 7 minutes. Add the onion and cook until soft, about 2 minutes. Add the garlic and red pepper flakes and cook, stirring, until the garlic begins to brown, about 2 minutes. Stir in the lentils, then add the tomatoes, 2 cups water, the basil and 1/2 teaspoon salt. Increase the heat to medium high and bring the sauce to a boil; cook 3 minutes. Reduce the heat to medium low and simmer, stirring occasionally, until slightly thickened, about 10 minutes.
Add the pasta to the sauce along with half of the parmesan. Season with salt and toss. Divide among bowls; drizzle with olive oil and top with the remaining parmesan and more basil.