Peel and thinly slice onion. Chop green onions. Mince garlic. Set everything aside.
In a small frying pan, drizzle 3 tablespoons of olive oil. Add in sliced onions and sprinkle with salt. Cook onions slowly with low heat. Stir constantly and cook until onions are softened and caramelized (about 20 minutes).
While onions are caramelizing, bring large pot of water to a boil. Season with salt and cook spaghetti as directed to al dente. If you prefer a firm pasta, cook pasta about 1 minute less than package directions. In a small bowl save 3-5 tablespoons of pasta water. Strain the pasta and set aside pasta and bowl of pasta water.
While onions and pasta is cooking, cook bacon. Time saving and hassle free tip: use precooked bacon and heat in the air fryer at 375F for about 6-8 minutes or until desired crispiness. Drain on paper towels and crumble into small pieces. Set aside.
Once onions are done caramelizing, pasta is cooked, and bacon is cooked and crumbled, add 3 tablespoons of salted butter to a large pan. Heat pan using medium heat and add in minced garlic.
Once minced garlic is fragrant, add in miso and 3 tablespoons of pasta water. Mix everything together and then add in spaghetti.
Mix in caramelized onions, green onions, and bacon. Cook until green onions are soft (about 1 minute).
Serve with a sprinkle of grated parmesan cheese.