Bring a large pot of salted water to a boil. Add the spaghetti and cook as the label directs. Reserve 2 cups cooking water, then drain.
Meanwhile, heat the olive oil in a large skillet over medium-high heat. Crumble the sausage into the skillet and cook, breaking it up with a wooden spoon, until lightly browned and no longer pink, 4 to 5 minutes. Clear a space in the pan, add the garlic and cook until just golden, 2 to 3 minutes. Add the cauliflower and cook until the edges are browned, about 2 minutes. Add 1 cup of the reserved cooking water, cover and reduce the heat to medium. Cook until the cauliflower is tender, about 8 more minutes. Uncover and boil over high heat until the liquid is almost evaporated, about 2 more minutes.
Add the spaghetti to the skillet along with the scallions. Drizzle with olive oil and season with salt. Toss for a minute or two to wilt the scallions and coat the pasta with the sauce, adding up to 1 cup cooking water, if needed, to loosen. Remove from the heat, sprinkle with the cheese and toss. Divide among shallow bowls and drizzle with more olive oil, if desired.
Photograph by Antonis Achilleos
Courtesy of Food Network Magazine