Heat oil in a Dutch oven over medium-high heat; add onion and garlic, and sauté 5 minutes. Add tomato, and reduce heat to medium.
Simmer 20 minutes or until tomato is soft. Add basil, and simmer 7 to 8 minutes more. Let cool 10 minutes.
Puree in a food processor 1 minute or until smooth. Add salt; cover and chill 1 hour.