Heat olive oil in a large deep skillet or pot over medium heat.
Add the sliced onions and 1/2 teaspoon salt. Cook, stirring occasionally, for about 20 minutes until the onions are deeply caramelized and golden brown.
Add the minced garlic and cook for 1 minute until fragrant.
Pour in the vegetable broth and water, then add the dried spaghetti pasta. Stir to combine and ensure the pasta is submerged in the liquid.
Bring the mixture to a boil, then reduce heat to medium-low and simmer uncovered.
Cook, stirring occasionally, for 12-15 minutes or until the pasta is tender and most of the liquid has been absorbed.
Stir in the butter, grated Parmesan cheese, remaining 1/2 teaspoon salt, and 1/4 teaspoon black pepper until the cheese and butter have melted and the sauce is creamy.
Remove from heat and sprinkle with chopped fresh parsley before serving.