In a large pot, heat the 2 tablespoons of olive oil over medium heat. Add the 1/2 cup of diced onions and sauté until they become translucent, about 4-5 minutes. Then, add the 3-4 diced garlic cloves and continue to sauté for another minute until fragrant.
Pour in the 28 ounces of tomato puree into the pot, stirring well to combine with the onion and garlic mixture.
Sprinkle in the 2 teaspoons of Italian seasoning, 1 teaspoon of kosher salt, and black pepper to taste. If you enjoy a bit of heat, add the optional 1/4 teaspoon of red pepper flakes. Stir everything together to ensure the spices are evenly distributed throughout the sauce.
Allow the sauce to come to a gentle simmer. Reduce the heat to low and let the sauce cook for about 30 minutes, stirring occasionally.
If you prefer a smoother sauce, now is the time to use a blender. Carefully transfer the sauce into a blender and pulse until it reaches your desired consistency. Return it to the pot and give it another quick stir.
Once your sauce is done cooking, remove it from the heat and allow it to cool. This sauce can be used immediately or stored in an airtight container in the refrigerator for up to a week, or in the freezer for up to three months.