If you're using fresh tomatoes, wash them and roughly chop them into a dice (it doesn't have to be small, just a rough dice is fine). You don't need to peel the tomatoes.If using canned tomatoes you don't have to prepare them at all. Heat a heavy based pan (a Dutch Oven is perfect) and add olive oil and diced onions to the bottom of the pan. Fry at medium heat until the onion start to brown and soften (around 5 to 10 minutes).
Add the fresh or canned tomatoes (whichever you use) and the finely chopped garlic and stir through to fully combine with the olive oil and onions. Allow to cook for around 5 minutes on medium heat, stirring through once or twice.
Now add the sugar, herbs, pepper and salt and stir really well to ensure the sugar is well dissolved. Then add the tomato paste and stir again.
Simmer the sauce on a low heat for around 1 hour. Leave the lid off to help the sauce thicken. Stir occasionally to ensure the bottom doesn't catch.
Once the sauce has thickened and cooked for around an hour, take it off the heat.
Use an immersion blender or place into a food processor and blitz the sauce until it's thick and smooth (you can make it as chunky or smooth as you like). I generally do this in my Thermomix (a type of food processor).
Place the finished sauce into a bowl or jar and into the fridge to chill before you use it on a pizza (hot sauce will make the base soggy).