Preheat oven to 425 degrees F. Slice plums in half and remove pits. Toss the plums with lemon juice in a large bowl; add sugar and toss once again. Arrange the plums cut-side up in a shallow baking dish. Roast for 20 to 25 minutes, or until tender. If the juices in the pan begin to burn, add a little water and cover the pan loosely with aluminum foil. Let cool slightly before serving.