1 pound collard greens, (about 12 cups), stripped from thick stems, washed, dried and coarsely chopped (1/2-inch pieces)
2 ounces sliced pancetta, or bacon, finely diced (3/4 cup)
2 teaspoons extra-virgin olive oil
1 medium onion, chopped
2 cloves garlic, minced
1/8 teaspoon crushed red pepper
1 28-ounce can diced tomatoes, (not drained)
1/4 cup water
8 ounces medium pasta shells, (3 cups)
1/4 teaspoon salt
Freshly ground pepper, to taste
1/2 cup freshly grated Parmigiano-Reggiano cheese
Bring 2 cups lightly salted water to a boil in a large wide pan. Add collards and cook until tender, 10 to 12 minutes. Drain, rinse with cold water and press out excess moisture. Set aside.
Put a large pot of lightly salted water on to boil for cooking pasta.
Cook pancetta (or bacon) in a large nonstick skillet over medium heat, stirring often, until golden, 5 minutes. Drain; discard fat.
Add oil to the pan and heat over medium heat. Add onion and cook, stirring often, until softened, 3 to 5 minutes. Add garlic and crushed red pepper; cook, stirring, for 30 to 60 seconds. Add the pancetta (or bacon), tomatoes and water; bring to a simmer, mashing the tomatoes with a potato masher or the side of a wooden spoon. Reduce heat to medium-low and simmer, uncovered, until thickened, about 20 minutes.
About 10 minutes before the sauce is ready, cook pasta in the boiling water, stirring often, until just tender, 8 to 10 minutes. Reserve 1/4 cup of the cooking water and drain the pasta.
Add the pasta, collards and reserved pasta-cooking water to the tomato sauce. Heat, stirring, until the pasta has absorbed some of the flavors, about 1 minute. Season with salt and pepper. Spoon into pasta bowls, sprinkle with cheese and serve.