Bring a large pot of water to a boil, salt it, add the pasta and cook until al dente. Set aside a ladle of pasta water and drain the pasta.
Meanwhile, heat the EVOO, three turns of the pan, in a large, deep skillet over medium heat. Add the garlic and cook for 3 minutes, then add the onion and cook for 5 minutes. Add the cauliflower, broth and rosemary, then cover and cook for 10 minutes.
Mash the cauliflower with the back of a spoon and cook until golden and caramelized, 7-8 minutes. Add the reserved ladle of pasta water to the cauliflower sauce. Stir in the pasta, then add the cheese and toss. Season to taste with salt and pepper. Divide the pasta among shallow bowls and top with the arugula.