Add olive oil to a 12-inch high-sided skillet and set over medium-high heat. Allow the oil to warm until it shimmers, indicating that it’s ready for the next step.
Once the oil is hot, add the onion and sauté just until they start to soften, turning translucent in color, about five minutes. You'll notice the sweet smell of the onions filling the room.
Add the garlic and crushed red pepper flakes. Sauté until the garlic is fragrant, about one minute. Be careful not to burn the garlic, as it can turn bitter.
Stir in the tomato paste. This step is crucial as it intensifies the flavor of the sauce. Mix well, ensuring the paste coats the onions and garlic.
Pour in the canned peeled whole tomatoes, including the juice. Use a wooden spoon to break up the tomatoes gently, creating a chunky texture.
Add the sprig of fresh basil, Italian seasoning, and kosher salt. Stir to combine all the ingredients, allowing them to meld together beautifully.
Reduce the heat to low and let the sauce simmer for twenty minutes, gently breaking up the tomatoes as they cook. You’ll want the sauce to thicken, transforming into a rich, hearty consistency.
After twenty minutes, the sauce should have thickened nicely. Remove the sprig of basil, and it’s now ready to be used as desired. You can serve it immediately or let it cool.
Store any cooled sauce in a plastic container or glass jar in the refrigerator for up to three to four days. It’s perfect for a quick meal later in the week!