Preheat the oven to 400 degrees.
Heat the olive oil in a skillet. Add the garlic, saute for a few seconds until it just starts to turn color, then add the cabbage and leeks. Season with salt and pepper and cook for 5-8 minutes or until the vegetables are just tender but not mushy. Turn off the heat
In a blender, place the cashews, cornstarch, nutmeg and salt and ground black pepper to taste. Add 3/4 cup water and blend into a very smooth paste. Set aside.
Roll out the puff pastry sheet to fit into your tart pan. I used a rectangular tart pan that's 12 inches by 8 inches, but a round pan would do just as well.
Place the puff pastry into the tart pan and press into the sides, making sure it fits into all the corners and crevices. Prick the pastry sheet all over with the tines of a fork, which will keep it from puffing too much. Don't remove any overhanging dough yet.
Arrange the slices of potatoes in the bottom of the tart pan, then layer on the cabbage and leeks. Finally, pour the cashew sauce all over. Once you've filled the tart pan, pinch off any excess dough along the edges of the pan.
Place the tart pan in the preheated oven, preferably on a baking sheet, and bake 30-35 minutes or until the puff pastry is golden-brown and the filling is set.
Let the tart stand 10 minutes before serving.