Put the 2 tablespoons olive oil, 155 grams onion, and 15 grams garlic in a pan and sauté over medium high heat while stirring constantly until the onions are a medium brown and have reduced to about half their original volume (about 5-7 minutes).
Add the 800 grams whole stewed tomatoes, 2 tablespoons tomato paste, 15 grams basil, ¼ teaspoon ground black pepper and ¼ teaspoon ground black pepper. Turn down the heat to medium low. Simmer for 10-20 minutes or until the sauce has the consistency you're looking for.
Taste the sauce. If it tastes a little bland and lacking in umami, try adding more salt. If you find the sauce is tasting too sour, try adding up to two teaspoons of sugar.
Notes
How long you need to simmer the tomato sauce will depend on your personal preference as well as what you plan to use it for. If you're using this in something that will be cooked again, like an eggplant parmesan, you don't need to get it that thick. If you're using this as a pasta or dipping sauce, you may want to get it a little thicker.