1. Preheat the oven to 350°F and bake for 15 minutes, until lightly golden. Set aside.
2. Combine ground almonds, sugar, egg, yogurt and flavoring if using, and mix.
3. Rinse and dry the plums and cut them in halves, discarding the stones.
4. Spread almond cream evenly over the partially baked tart shell, and arrange the plums on top in a circular pattern starting from the outside.
5. Spread brown sugar on top of the plums and drizzle with the lemon juice.
6. Bake for 30 minutes or until the plums are caramelized and bubbly and the walnut cream is set. If the side of the tart gets too brown, cover with foil.
7. Cool on a wire rack. If it is not eaten the same day of baking, cover with plastic or aluminum foil and keep refrigerated. Bring to room temperature before serving.