Finely chop tomatoes in a food processor.
In a medium saucepan, heat oil over medium heat. Cook onion until softened, about 10 minutes. Reduce heat to low and add tomatoes, jalapeños, salt, and 1/2 cup water. Cook, uncovered, until sauce has thickened and lightened in color, about 45 minutes. If sauce is chunky, return to food processor and whirl until finely chopped.
Note: Nutritional analysis is per tablespoon.