salt for the pasta water
1 pound ziti
1 pound sweet Italian sausage, without fennel seeds
1 large fennel bulb, about 1 pound
1/3 cup extra-virgin olive oil
2 medium onions, cut in half-moon slices
1/2 teaspoon salt
1/2 teaspoon hot red pepper flakes
1/2 cup tomato paste
3 cups finely chopped fennel fronds
1 cup freshly grated pecorino, or Grana or Parmigiano
Heat a large pot of salted water to boiling for the pasta. Drop the ziti into the boiling water, then bring back to a boil and cook until the ziti are not quite al dente. Drain, reserving 3 cups of the pasta water. While the pasta is cooking, remove the sausage from its casing and break the meat up a bit with your fingers. Trim the fennel bulb, removing any tough outer parts, reserving the fronds. Slice the bulb in half lengthwise, remove core then slice each half in ¼- inch- thick lengthwise slices.
Separate the slivers of fennel if they are attached at the bottom; cut the long slivers in half so you have about 3 cups of 2- inch- long matchsticks of fennel. Heat the olive oil in a skillet set over medium- high heat. Add the sausage meat, and cook, stirring and breaking it up more with a wooden spoon, until it sizzles and begins to brown, about 1½ minutes. Push the sausage to the sides of the pan, and drop the onion slices into the clear part of the pan; sauté, stirring, until they sizzle and wilt, another 2 minutes or so, then stir them in with the meat. Clear a new space, and drop in the fennel; let it heat up and wilt for 1 minute or more, then stir to combine.
Sprinkle on ¼ teaspoon salt; drop the hot red pepper flakes into a cleared hot spot, and toast the flakes for ½ a minute, then stir to combine. Clear a good sized hot spot in the center of the pan, plop in the tomato paste, and cook, stirring it in the spot, for a good minute of more, until it is sizzling and caramelizing. Then stir it in with everything else.
Ladle the reserved pasta water into the skillet, stir well, and bring the liquid to a boil. Reduce to a simmer, and let cook until the flavors have developed, the sauce is thickened but not too thick, and the fennel is soft but not mushy, 6 minutes or more. (Add more water if the sauce reduces too rapidly.) Season to taste.
Drop the cooked ziti into the simmering sauce. Toss everything together, then sprinkle over it the chopped fennel fronds, and continue to cook until the ziti are perfectly al dente and cooked with the sauce.
Remove the skillet from the heat, sprinkle the grated cheese over the ziti, and toss it in. Serve the hot pasta right from the skillet into warm pasta bowls.