In a medium sauce pan heat the oil or water on medium heat. Add the onions and stir to distribute evenly in the pan. Cook for approximately five minutes, stirring frequently, until the onions begin to caramelize. Add the garlic and stir. Cook for two minutes. Add the strained tomatoes, Italian seasoning, sugar, salt, and pepper and stir to combine. Bring to a boil. Reduce the heat to a low simmer and cook for twenty minutes. Stir frequently so the sauce doesn’t scorch.