Heat large pot salted water to boiling, drop in ziti until nearly cooked through. Drain and reserve 3 cups of pasta water. While pasta is cooking remove sausage from casing, break up "meat", trim the fennel bulb, reserve fronds. Slice the bulb lengthwise about 1/4 inch slices.

Separate the slivers of fennel into 2 inch matcchstick pieces. Heat olive oil in skillet over medium high heat. Add the sausage, stirring and breaking up with w wooden spoon until it sizzles and begins to brown. Drop in onion slices and saute in the same pan with they wilt about 2 minutes and then drop in fennel til it wilts for about 1 minute.

Sprinkle on 1/4 teaspoon salt add tomato paste until carmelizing. Then stir everything together. Ladle in saved pasta water and bring to a boil. Reduce and let cook till sauce thickens a little. Drop in Ziti. Mix and then sprinkle in cheese.