Bring a large pot of heavily salted water to a boil for cooking the pasta.

While water heats, add olive oil to a large, heavy-bottomed skillet over medium heat. Add sliced onions and a pinch of salt, stirring to coat evenly with oil.

Reduce heat to medium-low and cook onions slowly, stirring every 5-7 minutes, for 30-40 minutes until deeply caramelized and golden brown. If onions stick to the pan, add a splash of water to deglaze.

When onions are nearly finished caramelizing, cook pasta according to package directions until al dente. Reserve 1/2 cup pasta cooking water before draining.

During the last 2 minutes of caramelizing onions, add minced garlic and cook until fragrant, stirring constantly to prevent burning.

Add cooked, drained pasta directly to the skillet with caramelized onions. Toss to combine, adding reserved pasta water as needed to create a silky sauce that coats the pasta.

Remove from heat and stir in chili oil and red pepper flakes, adjusting the amount based on your preferred spice level. Add butter if using for extra richness.

Season with salt and black pepper to taste, then add half of the grated Parmesan cheese and toss until melted and incorporated.

Transfer pasta to serving plates or bowls. Top with remaining Parmesan cheese, fresh chopped parsley, and an additional drizzle of chili oil.

Serve immediately while hot, with extra Parmesan cheese and chili oil on the side for guests to customize their portions.