Preheat the oven to 350°F (180°C, or gas mark 4). Add the walnuts and pistachios to a food processor. Pulse until it reaches coarse crumbs, and then remove the blade of the food processor. Add the maple syrup, cinnamon, and rose water (if using) to the nuts and mix with a spoon. Set aside.To make the base: Line a tart pan with parchment paper. If your filo pastry is rectangular, cut it in half along the width. Place one layer of filo pastry in the tart pan, allowing the ends of the pastry to protrude from the pan. Brush the pastry with oil and repeat with half of the pastry to cover the entire tart pan. Scoop a few tablespoons (45 to 55 g) of the baklava mixture on the base of the tart and spread it evenly. Layer and repeat with the remaining pastry. Bake the pastry in the oven for 10 to 15 minutes or until the middle of the tart is golden.To make the custard: Add the cornstarch to a medium-size, deep saucepan with just enough soymilk to cover. Whisk until there are no more lumps of cornstarch. Add all the remaining custard ingredients to the saucepan and place it over high heat. Bring the custard to a boil for 3 minutes while whisking to make sure it doesn’t stick to the bottom of the pan. Reduce to medium-low heat and whisk for around 10 minutes or until it reaches a thick custard consistency. If there are lumps in your custard, blend it with a stick blender until it is smooth. Set aside the saucepan to cool for about 15 minutes.To assemble: While the custard is warm (but not hot), slowly pour it into your tart base. If needed, smooth the surface with the back of a spoon. Leave the tart at room temperature for about 30 minutes and then set aside in the fridge for at least 2 hours or overnight.To serve: When the custard has fully set, sprinkle with the remaining baklava topping and drizzle with additional maple syrup.Tip: Don’t worry if your filo pastry breaks. When you layer all your filo pastry, you won’t notice! After you pour the custard into the tart, make sure the tart doesn’t change temperature dramatically; otherwise, the custard will crack.