Bring a large pot of salted water to a boil. Add the pasta and cook for a few minutes less than the recommended time on the box.
Meanwhile, cut off and reserve the Swiss chard stems. Chop the Swiss chard leaves coarsely. Cut the stems into half-inch lengths.
In a large skillet over medium-high, heat the oil. Add the sausages and cook, stirring, for three minutes, or until lightly browned. Use a slotted spoon to transfer the sausages to a bowl.
Return the skillet to medium heat and add the onion. Cook until golden, three to five minutes. Add the garlic and cook, stirring, for one minute.
Add the Swiss chard stems to the skillet, cover and cook for three minutes. Remove the lid and add half the Swiss chard greens. Stir and cook, stirring occasionally, until the greens are slightly wilted. Add the remaining greens, cover and cook, stirring occasionally, until completely wilted. Add the tomatoes, a hefty pinch of salt and the red pepper flakes, if using.
When the pasta is almost done but not quite al dente, drain it, reserving one cup of the cooking liquid. Add the pasta and three-quarters cup of the cooking liquid to the skillet and simmer for three to four minutes, adding more cooking liquid if necessary, until the pasta is al dente and most of the liquid has evapourated. Add the sausages, then season with salt. Divide the mixture among four shallow bowls and top each portion with some of the cheese.