In a sauce pot, heat the extra-virgin olive oil over medium heat.
Add the diced onions and minced garlic and stir. Cover the pot and let the onions and garlic sweat in the olive oil until translucent and soft. Stirring every minute to ensure that they do not begin to burn. Lower the heat if needed. Cooking for about 3 minutes.
Add the anchovy paste to the cooked onions and garlic. Stir and cover the pot for another minute. The mixture should have a nearly paste-like consistency.
Add the crushed tomatoes to the pot and season with Kosher salt. Stir to combine well.
Add the fresh basil and bring the pot to a low simmer.
Simmer for 25 minutes, reduce heat as needed.
If using the same day, turn off the heat and let the pot sit on the cooktop until ready to use. Otherwise, use immediately or allow to cool and refrigerate for up to one week.