Heat up a large pot of water and prepare a bowl with iced water. Bring the water to boil and set iced water bowl close to the stove.
Slice a small x across the bottom of each tomato (not the part that attaches to the vine).
Working in batches of 3 or 4 tomatoes, add them to the boiling water for 25-30 seconds. You will see the skin starting to peel off where it's sliced.
Use a slotted spoon to take tomato out of the pot and into the ice bath. Once cooled after a few seconds, take them out onto the cutting board and peel the skin off with your hands. It should come off nice and easy.
To Make Pomodoro Sauce:
Dice onion and tomatoes and set aside. If you see a lot of juices running off the tomatoes, you can pour off some of them but not all. Peel, smash, and mince garlic cloves.
Preheat a Dutch oven over medium heat and add the olive oil.
Add onion and sauté until transparent and starts to soften. Make a little well in the middle and add garlic. Cook garlic just until fragrant.
Add in diced tomatoes and bring it all to a simmer. Lower the heat to medium low so the sauce gently simmers and let it cook for 30-40 minutes. Do not cover with a lid but make sure to stir from time to time.
Stir in salt, pepper, sugar, and minced basil leaves. Let the sauce cook another 5 minutes or so and then you can transfer it into a blender to blend until smooth. You can also use an immersion hand blender to blend the sauce.
Storing:
If not using the sauce right away, make sure to cool to room temperature before placing into the refrigerator.
Store Pomodoro sauce in a glass jar with an air-tight lid, in the refrigerator. Properly stored, the sauce should last 5-7 days.
Note: to help keep your sauce from spoiling faster, it's best to pour the sauce out of the jar instead of using a spoon to scoop it out.
Freezing:
To freeze the sauce, transfer cooled sauce into freezer friendly zip-top storage bag. (You can portions it into more than one bag.) Make sure to only fill it about 3/4 of the way full to allow room for the sauce to expand while it freezes. Seal well, label, and freeze for up to 3 months.