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