In a saucepan (at least 3 qt) on medium heat, add your olive oil and onions, stirring until the onions have soften. Once soften, add in your garlic and cook until fragrant.
Next, stir in your tomatoes, tomato paste, and basil. Bring the pot to a boil before lowering the heat to simmer the sauce. Stir on occasion, breaking up the tomatoes with a wooden spoon on the side of the pot. Be sure to give the sauce a taste half way to see if it needs any additional salt.
After around 20 minutes, your sauce should have thickened. If you want a smoother sauce, you can use an immersion blender on the sauce.