How to Make It
In a large pot of boiling salted water, cook the linguine until just barely al dente. Drain, reserving 1/2 cup of the pasta cooking water.
Meanwhile, in a large, deep skillet, heat 1/4 cup of the olive oil until shimmering. Add the sliced onion, thinly sliced garlic and crushed red pepper and cook over high heat until softened, about 2 minutes. Add the mussels and cook, tossing, until half of them are opened, 2 to 3 minutes. Add the coarsely chopped kale leaves and cook, tossing, until wilted, 3 to 4 minutes. Add the linguine and the reserved cooking water and cook, tossing, until the linguine is al dente, the mussels are open and the sauce is thickened, about 2 minutes longer. Discard any unopened mussels. Season the linguine generously with salt. Transfer the linguine to shallow bowls, drizzle each with 1/2 tablespoon olive oil and serve at once.