Set a pot of water to boil for the pasta.
In a deep pot, set on low-medium heat, add in the vegetable oil and the sliced onions. Season with 1 teaspoon of salt. Stir frequently and allow to cook for 20 minutes. Onion slices with shrink, release water then gradually gain brown coloring. Onion slices are properly caramelized with they stop releasing moisture and have a light brown color. Make sure you keep your eye on it. Onions go from caramelized to burnt quickly
Once onions are cooked, add in the garlic. Cook for a minute until garlic is fragrant then add tomato paste.
Stir tomato paste into the onion-garlic mixture. Cook for a few minutes until it changes color from bright red to brownish-red. Then pour in the crushed tomatoes. Cook until the sauce is reduced again and loses moisture completely. Take off heat.
Cook pasta according to directions. Reserve a 1/2 cup of pasta water then drain the rest.
Pour the cooked pasta into the tomato sauce. Add 1/4 cup of pasta water to loosen up sauce and dress the pasta. Taste the pasta and adjust salt if needed. You can add more pasta water if needed. Enjoy.