Trim stems from fennel bulb and cut away any brown spots from outer layer. Chop and reserve 2 tablespoons fronds and chop bulb. Saute fennel bulb and onion in oil in a 12-inch heavy skillet over moderately high heat, stirring, until beginning to brown. Add sausage and cook, stirring and breaking up lumps with a fork, until no longer pink. Add wine and simmer until reduced by about half, then add marinara sauce and simmer, stirring frequently, until vegetables are tender and sauce is thickened, about 10 minutes.
While sauce simmers, cook pasta in a pot of boiling salted water until al dente, 8 to 10 minutes. Drain and toss with sauce. Sprinkle with reserved fennel fronds.