To make the stuffing, preheat the oven to 200°C/fan 180°C/gas mark 6. Heat the olive oil in a small frying pan over medium-low heat and cook the shallots for 6-8 minutes until soft. Add the garlic in the final minute.
Tip the shallots and garlic into a medium bowl and leave to cool slightly, then add the sausages, breadcrumbs and sage. Mix together with your hands, then press into a small baking tin. It should be about 2cm high.
Bake in the oven for 20 minutes until cooked through. This makes about 400g of stuffing, but you will only need 60g for the recipe. The leftover stuffing freezes well and is also excellent in a sandwich.
To make the pizzas, bring 375ml water to the boil and slowly pour in the polenta, stirring continuously. Continue to stir for about 1 minute, until the polenta is thick. Tip out onto a board and divide into four.
Roll out each piece into a rectangle, about 25cm long and 18cm wide, between two sheets of greaseproof paper. Using a sharp, small knife cut out the shape of a large Christmas tree.
Bake the trees in the oven for 15 minutes, then remove. Top each one with the turkey, stuffing, tomatoes, mozzarella and drizzle with olive oil. Lay the asparagus or spring onions on to the bottom of the tree to look like a trunk. Return the pizzas to the oven for a further 5 minutes, until the mozzarella has melted and the base is crisp.
Top with the shredded sage and enjoy immediately.