Add the chopped onion and garlic to a pot on a medium high heat. (either a dry pot or with a little oil) Stir regularly. If not using oil, once the onion and garlic begin to sizzle and stick to the sides of the pot, add a little water to loosen and prevent burning. 

Add the chopped carrot, chopped pepper, chopped celery, and dried oregano and stir for another few minutes. 

Add the chopped tomatoes to the pot. 

Bring to a boil, then turn down to a gentle simmer for 20 minutes, stirring regularly. 

Blend the sauce with a hand blender to the desired consistency. 

Add salt and pepper to taste.