Bring a large pot of salted water to boil. Add pasta and cook until al dente. Drain, reserving at least ½ cup of the pasta water for sauce.
Meanwhile, in a large skillet or dutch oven, cook pancetta over medium high heat until browned, about 4-5 minutes. Add in fennel and onion and cook until they soften, about 5 more minutes. Add olive oil, toss to coat. Next, add in red chili flakes and cabbage, working in batches if needed. Cook cabbage for 10-12 minutes, until the cabbage has softened and started to brown.
Reduce heat to low, add butter and toss to coat. When pasta is ready, add it to cabbage. Pour in pasta water, 1-2 tablespoons at a time, stirring constantly, until a nice sauce starts to form. Add in ¾ cup Parmesan and toss to coat once more.
Season with remaining Parmesan cheese, salt, and pepper to taste.