Bring large pot of water to boil for pasta. Salt water and add spaghetti and kale, boil 5 minutes and drain. Return pot to stove, add wine and reduce over high heat for 2 or 3 minutes. Once boiling, add in the pasta and kale and cook until most of the liquid evaporates over medium-high heat, tossing frequently with tongs.
Meanwhile in a skillet heat 3 turns of the pan with olive oil over medium-high heat. Add onions, garlic and chili flakes, salt and pepper, cook till tender 7-10 minutes. Sprinkle with sugar. Add some of the wine sauce to deglaze the pan, then scrape into pot to combine with pasta. Toss with walnuts and some cheese and serve.