Cook pasta per pkg. directions, stirring occasionally to prevent sticking. Reserve 1 cup cooking water; drain and rinse pasta.
Meanwhile, heat 2 Tbsp oil in large skillet on medium. Quickly add fennel seeds and bite-size pieces of sausage on top, gently pressing with spatula. Cook on medium-high, undisturbed, until golden brown on bottom, 4 to 6 min. Toss and cook just until cooked through, 1 to 2 min.; transfer to bowl. Add chicken broth (or water) to same skillet and bring to a simmer, scraping bottom of pan. Transfer to bowl with sausage.
Wipe out skillet and heat remaining Tbsp oil on medium. Add sliced fennel and ½ tsp salt and cook, stirring occasionally, until just tender, about 4 min. Stir in garlic and red pepper flakes and cook 1 min. Stir in lemon juice and ½ cup reserved cooking liquid.
Gently fold pasta and ¼ cup cheese into sliced fennel, then kale and half of sausage, adding more cooking liquid if pasta seems dry. Serve topped with remaining sausage and 2 Tbsp Pecorino Romano.
