Drain the soaked cashews from their soaking liquid. Discard liquid. Place soaked cashews in a food processor or high-speed blender. Pulse for a few seconds. Add the plant-based milk and pulse until smooth.
Add the brown sugar, agar flakes, nutritional yeast flakes, and ground nutmeg. Pulse for a second or two to combine.
Pour this into a medium-sized saucepan over medium heat. Bring to a boil. Reduce heat to simmer for 5 minutes.
Remove from the heat. Stir in the rum. Set aside for 5 to 10 minutes, to allow it to cool slightly.
In a separate saucepan add the caramel topping ingredients. Bring to a boil and reduce to a rapid simmer. Cook until the sugar caramelizes and the sauce thickens, 5 to 7 minutes. Pour this evenly in the bottom of two flan dishes.***
Pour equal amounts of the slightly cooled flan mixture over the caramel.
Place in the fridge for at least one hour, preferably two.
To serve, use a butter knife to cut along the sides of the pan. Invert it over a serving dish. It may take a minute or so, but the flan will drop onto the plate. Sprinkle with additional nutmeg (freshly grated is a nice touch).