In a large pot, heat your olive oil over medium heat. Add the onions and saute for around 5 minutes, until they start to become soft and translucent. Using a wooden spoon, stir in the garlic and saute for another 30-60 seconds.
Next, add the tomatoes, basil, salt, and pepper to the pot.
Turn the burner up to medium-high and allow the ingredients to come to a boil. Once it's boiling, turn the burner down to medium-low heat, cover the pot with a lid, and allow the sauces to simmer for at least 30 minutes, stirring occasionally.
Remove the tomato-based sauce from heat and allow it to cool slightly. Using an immersion blender, puree the sauce to your desired level of smoothness. (You could also pour the sauce into a food processor and create your tomato puree that way, but if you have a hand-held immersion blender, it's much easier to do it in the pot.)