Add the soaked and drained cashews to a high-powered blender. Add the coconut cream, agave, lemon juice, salt, ginger, cardamom, coconut oil, vanilla, and orange zest. Blend until smooth and creamy, stopping to scrape down the sides as you go. If using a food processor, blend the cashews first until broken down into tiny bits, then add the remaining ingredients; it will take about 4 minutes to fully blend.
Pour a little less than half of the filling (about 1 cup/240 g) into the cooled tart shell, using a silicone spatula to smooth the surface. Refrigerate for 20 minutes.
Peel the mangoes and slice into thin strips, about 1/8 to 1/4 inch (about 0.5 cm) thick.
Remove the tart from the fridge and top with half of the sliced mango, arranging it in a flat layer. Pour the remaining filling on top, smooth out, and refrigerate for 20 minutes.
Top the tart with the remaining mango in a flat layer, or in fancy curved lines as shown. Refrigerate at least 2 hours or overnight (when chilled longer, it has a firmer custard-like texture).
Before serving, top each slice with a few raspberries and mint leaves (if using). Refrigerate leftovers for 3 to 5 days.