Heat olive oil in a large Dutch oven or stainless steel pot over medium heat. Add the diced onion and kosher salt. Cook for 5-7 minutes until the onion is soft and just starting to turn golden.
4 tablespoons extra virgin olive oil, 1 large yellow onion, 1 teaspoon kosher salt
Stir in the minced garlic, Italian seasoning, and red chili pepper flakes. Cook for an additional 1-2 minutes to release the flavors.
4 cloves garlic, ½ tablespoon Italian seasoning, ¼-½ teaspoon red chili pepper flakes
Add the canned San Marzano tomatoes to the pot. Use a wooden spoon to start breaking up the tomatoes to release their juices.
2 28-ounce cans San Marzano whole peeled tomatoes
Bring the sauce to a gentle simmer, then reduce the heat to low. Cover the pot with a lid and let the tomatoes simmer for 20-30 minutes or until they are tender.
Once the tomatoes are tender, you can blend the sauce using an immersion blender directly in the pot or carefully transfer the sauce to a regular blender and puree it in batches. Alternatively, use the back of a wooden spoon to crush the tomatoes for a chunkier texture.
If you prefer a thicker tomato sauce, heat it back up to a gentle simmer after pureeing. Cook uncovered for an additional 20 minutes or until the sauce reaches your desired thickness. Keep in mind that longer simmering will result in a thicker sauce.
Taste and adjust the seasoning if needed. Serve with pasta, pizza, meatballs, or as a marinara dip. Enjoy!