In a frying pan heat olive oil; add onion and the garlic. Saute until brown and remove the garlic.
Put the canned tomatoes and the liquid from the can in a blender and mix gently, less than a minute.
Slowly pour the tomatoes into the frying pan. Be very careful because the liquid on the oil has a tendency to explode. Let this simmer for about 15 minutes. Add the seasoning, stirring well.
In the meantime, put 3 tablespoons of salt into a 4 quart pot of water and bring to a boil. When the sauce is almost ready, put the spaghetti into the boiling water. Stir occasionally and when the spaghetti is almost done, pour in a colander and immediately transfer to a heated platter.
Pour the sauce over the the pasta and mix a little with 2 forks. Sprinkle with some chopped parsley and serve. Sprinkle with cheese,if desired, on top.