Heat olive oil in a large skillet over medium-low heat. Add sliced onions and toss to coat.

Cook onions slowly, stirring occasionally, for 30-40 minutes until they are deeply browned, soft, and caramelized. Add a pinch of salt halfway through.

While onions finish caramelizing, boil pasta in heavily salted water until just shy of al dente. Reserve 1/2 cup of starchy pasta water before draining.

Push caramelized onions to the edge of the skillet. Add butter and minced garlic to the center. Sauté garlic for 1 minute until fragrant.

Stir the garlic, soy sauce, balsamic vinegar, and chili oil into the onions to create the sauce base.

Add the hot drained pasta directly into the skillet along with 1/4 cup of the reserved pasta water.

Toss vigorously over medium heat until the starchy water and oils emulsify into a glossy sauce that coats the noodles. Add more pasta water if needed.

Remove from heat. Garnish with chopped parsley, coarse black pepper, and toasted sesame seeds. Serve immediately.