Sweat onions Heat oven to 275°F. Heat oil in a 4-quart Dutch oven over medium heat. Add onions and sweat until soft and translucent, about 8 minutes, stirring occasionally. Add tomatoes and salt and stir to combine.
Poach in oven Transfer the mixture to oven and cook (uncovered), gently stirring occasionally, until onions are light golden and completely tender (almost like a jam) and tomatoes are wrinkled but not mushy, 4 to 4 1/2 hours. The confit can be allowed to cool and then stored in an airtight container for up to 2 weeks in the refrigerator.
