Bring a large pot of salted water to a boil. Add the spaghetti and cook until al dente. Reserve 1/2 cup of the cooking water, then drain the spaghetti.
Heat the olive oil in a large skillet over medium heat. Add the onion and garlic and cook for 5 minutes, until the onions are softened. Add the red chard and cook for 5 more minutes, until the chard is wilted.
Stir in the mustard, cream, and Parmesan cheese. Cook until the sauce is thickened, about 3 minutes. Add the reserved cooking water to thin the sauce, if desired.
Add the cooked spaghetti to the skillet and toss to combine. Season with salt and pepper to taste. Serve immediately.
Interesting Facts
Mustard is a great condiment to add to a variety of dishes, including this spaghetti.
Red chard is a type of leafy green vegetable with a slightly sweet flavor.
Parmesan cheese is a type of Italian cheese that is commonly used in pasta dishes.