Trim off tough stems and ribs from kale and coarsely chop leaves. In large pot of boiling salted water, cover and cook kale until tender, about 3 minutes. Drain and chill in cold water; drain again and squeeze dry. Set aside. (Make-ahead: Refrigerate in airtight container for up to 24 hours.)In nonstick skillet, heat oil over medium-high heat; saute potatoes until golden brown, about 15 minutes. Add pancetta, garlic, sage, salt and hot pepper flakes; saute for 3 minutes. Meanwhile, in large pot of boiling salted water, cook pasta until tender but firm, about 8 minutes. Drain and return to pot. Stir in kale, potato mixture, stock, Fontina cheese and lemon juice; heat through, tossing to coat, about 3 minutes.