Tip: Instead of grating the lemon for the gremolata, use a zester and then finely chop the long strips of zest with a chef’s knife. This zest is a little chunkier and easier to sprinkle.

Bring a large pot of well-salted water to a boil.

Meanwhile, heat the oil in a 10-inch skillet over medium heat. Add the onion and red pepper flakes, season with a big pinch of kosher salt, and cook, stirring frequently, until tender and nicely browned, about 15 minutes (reduce the heat to medium low if the onion is browning too fast). Add the chile and continue to cook for 1 minute. Turn off the heat, add the butter, and swirl the pan to melt. Add the lemon juice and another pinch of kosher salt. Keep warm.

Cook the linguine in the boiling water until al dente, about 10 minutes.

Meanwhile, make the gremolata: Combine the parsley, garlic, and lemon zest on a cutting board and chop them together with a chef’s knife until the parsley is finely chopped and mixed well with the lemon and garlic.

Drain the pasta and return it to its cooking pot. Over medium heat, add the onion mixture to the pasta and toss to combine. Add the 1/2 cup Pecorino, quickly toss again, and add salt to taste. Transfer the pasta to a platter or shallow bowls. Sprinkle liberally with the gremolata and more Pecorino and serve.