Heat oil in large nonstick skillet over medium-high heat. Add sliced fennel bulb, sausage, and mushrooms; saute until sausage is brown and cooked through and fennel is almost tender, 12 to 15 minutes. Add garlic and fennel seeds; stir 1 minute. Stir in cream, then 1 cup broth; boil until liquid is reduced and very slightly thickened, 2 to 3 minutes. Meanwhile, cook tortellini in large pot of boiling salted water until just tender but still firm to bite, stirring occasionally. Drain tortellini; return to same pot. Add sausage mixture to tortellini in pot. Toss over medium heat until blended. Add spinach; toss gently until spinach wilts. Stir in 1/2 cup cheese; add more broth by 1/4 cupfuls to moisten if dry. Season with salt and pepper, sprinkle with chopped fennel fronds, and serve, passing additional cheese.