Take a paring knife and mark/cut "X"s on the top of the tomatoes. Make the cuts about 2 inches wide.
Bring a large pot of water to the boil
Place the tomatoes into the boiling water for around 90 seconds to 2 minutes, or until the peel starts to come off.
Next, remove the tomatoes and allow them to cool so you can handle them. Then remove the peels. I like using a knife
Chop the onion and begin cooking it in a large pot on medium heat. Add olive oil to sautee the onions and begin creating the flavor base. Add a pinch of salt. Cook/sautee for 3 minutes.If you would like a stronger flavored tomato sauce you could add garlic. But only add the garlic once the onions have cooked and begun to soften or risk the garlic burning. Give the tomatoes a rough chop and add them to the cooked onions. Cook together for 10 minutes on medium heat. You can add a little bit of water at this point. If you add too much water by accident, you can always "cook" / "reduce" the sauce on the stovetop. The longer you cook the sauce the thicker and more reduced it becomes.
Finish the sauce by using a blender, immersion blender, or food processor. Blend/puree until smooth. This is where you can add any additional flavors you want. Oregano, parsley, basil. Just be careful to not add too much or risk overpowering the tomato sauce.
Add smooth tomato puree back to the pot and cook till desired consistency.
You could add the tomato paste at this point for deeper color or just cook the sauce until it darkens in color.
Add salt & pepper to taste