Trim fennel, reserving fronds and bulb. Cut fennel bulb in half lengthwise; cut each bulb half lengthwise into (1/4-inch-thick) slices. Cut bulb slices into 2-inch-long pieces. Chop fennel fronds to measure 1/3 cup.
Bring 6 quarts water and 2 teaspoons of salt to a boil in a large stockpot. Stir in pasta; partially cover, and return to a boil, stirring frequently. Cook 8 minutes or until the pasta is almost al dente, stirring occasionally. Drain pasta in a colander over a bowl, reserving 2 cups cooking water.
While pasta cooks, heat oil in a large Dutch oven over medium-high heat. Remove sausage from casings. Add sausage to Dutch oven; cook 2 minutes or until lightly browned, stirring to crumble. Push sausage to one side of pan. Add onion to open space in pan; cook 1 minute or until onion begins to soften. Stir onion into sausage. Push onion mixture to one side of the pan. Add fennel bulb to open space in pan; cook 1 minute or until fennel begins to soften. Stir fennel into onion mixture. Stir in pepper and 1/4 teaspoon salt; cook 1 minute. Move the sausage and fennel mixture to outside edges of pan, leaving an open space in center. Add 1/4 cup tomato paste to open space in pan; cook 1 minute, stirring constantly. Stir tomato paste into fennel mixture.
Add reserved 2 cups cooking water to pan; bring to a boil. Reduce heat, and simmer 6 minutes or until fennel is tender. Add fennel fronds and pasta; cook 2 minutes or until pasta is al dente, tossing to combine. Remove from heat; stir in cheese. Serve immediately.