Brush onion with oil and place cut side down on a nonstick foil lined baking sheet. Roast at 450F for 20 minutes, then flip over, brush with a little more oil, then roast until the onion "pops" and looks caramelized and ruffled like a rose.
Pulse tomatoes in a food processor until they resemble chunky salsa.
Place all ingredients in a saucepan and simmer over low heat until reduced by half, about an hour.
Puree sauce in food processor (in batches if necessary) and cool before using, or reduce further by simmering until it is your desired consistency.