Preheat the oven to 190C/375F/Gas 5.
Place the beetroot into a roasting tin, drizzle with oil and season with salt and freshly ground black pepper.
Transfer to the oven and roast for 45 minutes, or until softened. Set aside to cool, then cut into wedges and place into a large bowl.
Increase the oven temperature to 200C/400F/Gas 6.
Add the cheese, onion, chilli, cranberries and spinach to the bowl with the beetroot. Season well with salt and freshly ground black pepper and stir well.
Lay the pastry squares out on a non-stick baking sheet and divide the beetroot mixture equally among the squares.
Draw the pastry up around the edges to completely enclose the filling in a tight ball.
Brush the pastry all over with beaten egg and sprinkle over the poppy seeds.
Transfer to the oven and bake for 15 minutes, or until crisp and golden-brown.
Meanwhile, for the potatoes, heat an ovenproof dish until very hot. Add the oil and the par-boiled potatoes, season well with salt and freshly ground black pepper and stir.
Place into the oven and roast for 25 minutes, or until crisp and golden-brown.
In a bowl, mix together the butter and wasabi until well combined (you may not need all of the wasabi butter - save the rest for use in another dish.)
When the potatoes are cooked and you're ready to serve, add the wasabi butter to the dish with the potatoes and stir well to coat. Sprinkle over the coriander.
To serve, place a pie onto each serving plate with some wasabi potatoes and Brussels sprouts.