Preheat the oven to 180°C / 350°F (top and bottom heat).
Drain the mandarin oranges. (You can save the juice to drink or use it in another dessert—it’s sweet and delicious!)
Combine all dough ingredients—vegan butter/margarine, sugar, flour, ground almonds, and citrus zest—and knead into a dough.
Press about ⅔ of the dough into a small baking dish lined with parchment paper.
Distribute the drained mandarin oranges over the dough.
Crumble the remaining ⅓ of the dough over the mandarins as a streusel topping. Sprinkle with 1 tablespoon of sugar.
Bake in the middle of the oven for about 30 minutes. Keep an eye on it—if the streusel starts browning too quickly, move the cake to a lower rack and finish baking. It’s done when the streusel topping is golden brown.
Let cool slightly before slicing—best enjoyed warm or at room temp.