Roasting the tomatoes, pepper, and onions gives them a rich, caramelized flavor that stands up to the smoky taste of the chipotle.
Preheat oven to 500°.
Arrange the plum tomatoes, onion slices, and jalapeño pepper on a jelly-roll pan coated with cooking spray, and bake vegetables at 500° for 30 minutes. Remove jalapeño pepper from pan. Bake tomatoes and onion slices an additional 15 minutes or until the vegetables are soft. Combine the tomatoes, onion slices, jalapeño pepper, lime juice, salt, and chipotle chile in a food processor, and process until tomato mixture is smooth.