In a large skillet, cook the onion in the olive oil over medium-low heat for five minutes, or until soft. Add the crushed garlic, chili powder, cumin, and salt to the pan. Stir and cook for another minute to combine the flavors.
Add the tomato sauce to the pan. Simmer gently for 10 minutes, stirring occasionally.
Let the sauce cool, then transfer it to an airtight container. You can store it in the refrigerator for several days, or freeze it for several months.