Preheat oven to 425°F (220°C). In a small bowl, combine the ricotta, maple syrup and orange zest. Spread mixture on the flatbread. Set aside.
In a medium bowl, toss the plums, brown sugar, butter and cardamom to coat evenly. Arrange the plum slices on the flatbread and drizzle with any liquid left in the bowl.
Bake directly on middle rack of oven for 12 min., until golden. Cool slightly before slicing into 8 wedges. Sprinkle with thyme and serve warm or at room temperature.