Cook 1 Kg. tomatoes, 2 large onions and 15 garlic cloves in the pressure cooker with 3 whistles. Let it cool.
Make a puree in a blender.
strain It.
Simmer it on a low heat and reduce it to half.
Add Italian seasoning and chili flakes.
Chop remaining onion and tomatoes.
In a pot heat olive oil. Saute chopped onion and tomato until they are softened.
Mash them with masher.
Add the above puree.
Add sugar, salt. Let it simmer for 10 minutes on low heat.
Notes
You will get 3 cups of sauce from this recipe. You can store the extra sauce and use it afterwards.If you prefer fine sauce then skip the step of cooking additional tomato and onion and let everything cook in the pressure cooker and strain it and simmer it.