Ziti with Sausage Onions and Fennel

Ingredients: FOR THE SAUCE
1 pound sweet Italian sausage (without fennel seeds)
1 large fennel bulb with stem and fronds (about 1 pound)
1/3 cup extra-virgin olive oil
2 cups half-moon onion slices (about 2 medium onions)
1/2 teaspoon salt
1/2 teaspoon peperoncino flakes
1/2 cup tomato paste
Boiling water from the pasta cooking pot
FOR THE PASTA AND FINISHING
1 tablespoon kosher salt (for the pasta cooking water)
1 pound ziti
1/3 cup finely chopped fennel fronds
1 cup freshly grated pecorino (or Parmigiano-Reggiano or Grana Padano)

Directions: Heat 6 quarts of water with the tablespoon kosher salt to boiling in the pasta cooking pot.
Remove the sausage from its casing and break the meat up a bit with your fingers.
Trim the fennel bulb. Slice the bulb in half lengthwise, then slice each half in 1/4-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.
Have the remaining sauce ingredients ready and nearby.
MAKING THE SAUCE AND COOKING THE PASTA SIMULTANEOUSLY,
Pour the olive oil into the skillet and set it over medium-high heat. Add the sausage meat and cook, stirring and breaking it up more with a wooden spoon, until it is sizzling and beginning to brown, about 1-1/2 minutes.
Push the sausage a bit aside and drop the onion slices into a clear part of the pan; sauté, stirring, they're sizzling and wilting, another 2 minutes or so, then stir them in with the meat.
Clear a space and drop in the fennel; let it heat up and wilt for 1 minute or more, then stir it around with the sausage and onions.
Sprinkle on 1/4 teaspoon salt; drop the peperoncino in a hot spot and toast the flakes for 1/2 minute, then stir them in.
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 or more, until it is sizzling and caramelizing; then stir it in with everything else.
Ladle 3 cups of boiling pasta water from the pot into the skillet, stir well and bring the liquid to a boil. Adjust the heat to maintain an active simmer all over the pan.
Drop the ziti in the boiling water in the pasta pot. Stir and bring back to the boil.
Cook about 8 minutes (a minute less than what is recommended on the package)
until the ziti are not quite al dente.
Continue to simmer the sauce until the flavors have developed and the fennel is
soft but not mushy, 6 minutes or more. The sauce should not get too thick: stir in
another cup or 2 of boiling pasta water, if it reduces rapidly. When the sauce is
done, taste it and add more salt if you want. If the pasta is not ready, turn down
the heat to keep the sauce at a very low simmer until the ziti are on their way—
then turn the heat up.
As soon as the ziti are ready by your timing, lift them out of the pot with a spider. Let excess water drip off only for an instant and drop the wet cylinders into the simmering sauce.
Start tossing pasta and sauce together; ladle in more water if the sauce seems too thick.
Sprinkle over the chopped fennel fronds and continue to cook and toss the ziti in the skillet for 2 minutes or until they are perfectly al dente and coated with sauce. If the pasta appears dry, ladle in more hot pasta water; if it is soupy, cook rapidly to thicken the sauce.
FINISHING THE PASTA
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.