Coat a large skillet with nonstick cooking spray. Add onions and garlic; cook over medium-high heat 5 minutes, adding 2 tablespoons water halfway through. Stir in sausage, breaking up clumps; cook until no longer pink, 5 minutes, adding 1/4 cup of the broth halfway through.
Cook spaghetti following package directions. Drain.
While spaghetti is cooking, add mushrooms and 1/4 cup more broth to skillet; simmer 3 minutes. Add tomatoes, salt, fennel seeds and pepper; cook 3 minutes. Add remaining 1/4 cup broth and arugula; cook 2 minutes. Toss spaghetti with sauce. Serve warm.