Defrost the pastry squares at room temperature (it should take approximately 10 minutes). Preheat the oven to 200°C / fan 180°C / gas 6.
Using a small sharp knife lightly score a border around the pastry square 1cm from the edge. Spread the pasta sauce evenly over both squares of pastry, being careful not to go over the border.
Toss the chicken very lightly in 2 tsp of the chilli oil and scatter evenly over the tomato sauce-topped pastry squares. Scatter over the mushrooms and spring onion, again being careful not to go over the border, and then drizzle both tarts with the remaining oil. Whisk the egg yolk with 1 tbsp of cold water and then brush the border with the egg mixture.
Place on a baking tray and cook in the oven for 16-18 minutes or until golden brown. Lightly garnish with spinach leaves and serve.