01 - Bring a large pot of salted water to a boil. Cook fettuccine until al dente according to package directions. 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 for 10 minutes, stirring occasionally, until onions soften and begin to brown.
03 - Sprinkle brown sugar over onions. Continue cooking for 15-20 minutes, stirring frequently, until onions are deeply caramelized and golden brown.
04 - Stir in minced garlic and chopped red chili pepper. Sauté for 1-2 minutes until fragrant.
05 - Add 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. Add reserved pasta water one tablespoon at a time until sauce reaches desired consistency.
07 - Taste and adjust seasoning as needed. Serve immediately, garnished with fresh parsley and additional Parmesan cheese.