Several hours before making the tarts, place the cashews in a medium bowl and cover with 3 cups water. Soak the cashews for 4-6 hours, then drain and rinse. You'll also want to put the coconut milk (for the coconut whipped cream) into the refrigerator to chill.
To make the banana cream filling, place the soaked cashews into the pitcher of a blender or a food processor fitted with an S-blade. Add 2 bananas, 2 tablespoons lime juice, coconut milk or almond milk, coconut oil, syrup and salt. Puree until completely smooth, about 1 minute. Spoon the filling into a medium container with an airtight lid. Cover the top of the pudding with plastic wrap, taking care to expel any air bubbles - the less the filling is exposed to the air, the less brown it will turn. Cover with the lid and refrigerate until chilled, 1-2 hours.
Set the tart shells out. Spoon the filling into a pastry bag affixed with the tip of your choice or a zipper bag with the corner snipped off. Pipe the filling into the tart shells, dividing evenly between each shell (depending on your shells, you should be able to fill them almost to the top).
Place the coconut whipped cream into another pastry bag or zipper bag and pipe over the top of the banana filling. If you'd like to avoid the pudding turning brown on the top, take care to cover the filling entirely with the whipped cream, so that it's not exposed to the air.
Thinly slice the remaining banana and place it on a plate or in a bowl. Drizzle remaining 1 tablespoon lime juice over the tops of the bananas and toss gently. Top each tart with a banana slice.
*I like to make this with perfectly yellow bananas - not too ripe, with little to no brown spots. Prep time includes 6 hours soaking and chilling time. The tarts themselves take about 20 minutes to make and assemble.