Photograph: Martin Poole
Preheat the oven to 190°C, fan 170°C, gas 5. For the pastry, place the flour, xanthan gum, sunflower spread or margarine and vegetable fat in a food processor and pulse until the mixture is the consistency of breadcrumbs. Add 2-3 tablespoons of cold water, one at a time, pulsing as you go, until the mixture begins to pull together. Add the sugar and pulse until combined, then turn into a large mixing bowl and bring together into a ball. Knead for a few minutes until smooth and elastic to touch, then wrap the pastry in clingfilm and pop it in the fridge to rest while you prepare the streusel topping.
For the topping, sift the flour and bicarbonate of soda into a large mixing bowl, then stir in the sugar, oats and a small pinch of salt. Add the spread or margarine and, using your fingers, rub it in to the flour until you have soft clumps of a crumble-like mixture.
Remove the pastry from the fridge, shape into a ball and lay between two large pieces of clingfilm. Roll it out to about 4mm thick, then peel off the uppermost layer of clingfilm and turn the pastry into a 23cm loose-bottomed tart tin. Remove the clingfilm and gently press the pastry into the tin, pressing together any cracks or gaps that may appear with your fingertips.
Spoon the mincemeat into the case, spreading it evenly over the pastry using the back of the spoon. Crumble the streusel topping evenly over the mincemeat so it is all covered. Then using a pastry brush, lightly brush the pastry with oat or rice milk and bake for 35-40 minutes or until the streusel is a deep gold colour and the pastry is cooked through. Leave the tart to cool in the tin on a wire rack for 10 minutes before serving.