Preheat the oven to 180°C.
Cut a disc out of the polenta and slice into 2cm thick pieces.
Peel the sweet potato and do the same, ensuring that you have pieces of the same thickness.
Sweat the mushrooms, shallots and garlic in a pan until softened. Add the chopped walnuts and tarragon and stir.
Add the Parmesan and Gruyere cheeses to the béchamel sauce and stir until melted.
Assemble the stack within the disc cutter on a baking tray: place a disc of polenta at the base, followed by a disc of sweet potato, and add a layer of the mushroom stuffing mix. Press down firmly. Place another disc of sweet potato above the mushroom mix, and top with another disc of polenta.
Lay around 2 tablespoons of the béchamel sauce on top of the polenta, and crumble over the blue cheese to finish.
Bake for 40-45 minutes. To serve, place on a bed of wilted spinach and drizzle over any remaining béchamel sauce.