Roast the plums
Preheat the oven to 400°F(200°C). Line a rimmed baking sheet with aluminum foil. Place the plums, cut side down and spaced at least 1 inch (2.5 cm) apart, on the prepared baking sheet. Roast until the plums are tender and their juices begin to pool, about 10 minutes, depending on their ripeness.
Make the ginger syrup
Meanwhile, in a small saucepan over medium heat, stir together the ginger, sugar, and 2 tablespoons water and bring to a gentle simmer. Cook until the flavors have blended and the liquid has reduced to a light syrup, about 5 minutes.
Finish the dessert
Place the warm plums in bowls. Drizzle with the ginger syrup, top with the whipped cream, and serve.