How to Make It
In a large pot of salted boiling water, cook the pasta until al dente. Reserve 1 cup of the pasta water, then drain.
In the same pot, heat the olive oil until shimmering. Add the sausage and cook over moderately high heat, stirring occasionally, until just cooked through, about 8 minutes. Using a slotted spoon, transfer to a bowl. Add the leeks, garlic and a generous pinch of salt to the pot and cook over moderate heat, stirring occasionally, until the leeks are softened, about 5 minutes.
Return the sausage to the pot. Add the mustard greens and tomatoes and cook over moderately high heat, stirring, until the greens are just wilted and the tomatoes start to burst, 5 to 7 minutes. Add the pasta, crème fraîche and reserved pasta water and cook, tossing, until the pasta is hot and coated in a light sauce, about 2 minutes. Season the pasta with salt and pepper and serve right away.