Heat a large pot of salted water to boiling and add the ziti. Cook until not quite al dente. Drain reserving 3 cups pasta water.

Meanwhile, remove sausage from its casing and break up with your fingers. Trim the fennel bulb and remove the outer tough parts. Save the fronds. Slice the bulb in half lengthwise and remove the core. Slice each half into 1/4" by 2" slices and set aside.

Heat the olive oil in a very large skillet or pot and add the sausage breaking up further as it cooks for about 1 1/2 minutes. Do not cook the sausage through. Push the sausage aside and add the onion slices into the cleared part of the pan. Stirring until the sizzle and wilt about 2 min., stir into the meat.

Clear a new space and add the fennel and let it heat up for about 1 mn.

Clear another spot and toast the red pepper flakes for 30 seconds. Sprinkle with 1/4 tsp. salt and stir to combine. Now clear a big spot in the center of the pan and add the tomato paste. Cook stirring in that spot for 1-2 min. until it sizzles and caramelizes. Then stir into the rest of the ingredients.

Stir in the reserved pasta water, combining well and bring to a boil. Reduce to a simmer and let cook until the flavors have developed, and the sauce is thickened and the fennel is cooked about 6 min. Season to taste and add more water if it is too thick.

Add the cooked ziti and stir to combine and continue to cook until the pasta is al dente.

Sprinkle with the fronds and parmesan and serve right out of the pot or onto a warmed platter.