01 - Cook the fettuccine in a large pot of salted boiling water until al dente. Reserve 1/2 cup of pasta water, then drain the pasta and set aside.
02 - Heat olive oil and butter in a large skillet over medium heat. Add the sliced onions and salt. Cook, stirring occasionally, until onions soften and start to brown, about 10 minutes.
03 - Sprinkle in the brown sugar and continue to cook onions, stirring frequently, until deeply caramelized and golden, 15-20 more minutes.
04 - Add garlic and red chili pepper to the onions. Sauté for 1-2 minutes until fragrant.
05 - Stir in the honey, heavy cream, Parmesan, red pepper flakes, and black pepper. Simmer gently for 2-3 minutes until the sauce thickens slightly.
06 - Add cooked fettuccine to the skillet. Toss well to coat, adding reserved pasta water one tablespoon at a time until sauce reaches desired consistency.
07 - Taste and adjust seasoning as needed.
08 - Serve hot, garnished with fresh parsley and extra Parmesan cheese.