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, approximately 10 minutes.
03 - Sprinkle brown sugar over onions and continue cooking, stirring frequently, until deeply caramelized and golden brown, 15-20 additional minutes.
04 - Add minced garlic and chopped red chili pepper to the caramelized onions. Sauté for 1-2 minutes until fragrant.
05 - Stir in honey, heavy cream, Parmesan cheese, red pepper flakes, and black pepper. Simmer gently for 2-3 minutes until sauce thickens slightly.
06 - Add cooked fettuccine to the skillet. Toss thoroughly to coat, adding reserved pasta water one tablespoon at a time until sauce reaches desired consistency.
07 - Taste and adjust seasoning as needed. Serve hot, garnished with fresh parsley and additional Parmesan cheese.