Make the shortbread base by mixing the flour, xanthan gum if using and sugar together, then cutting in the margarine with a fork until the mixture resembles breadcrumbs

Press into the bottom of the tin and bake for 25 minutes. It won't be completely done at this point but don't bake any longer

Meanwhile put the diced pears in a pan and cover with water. Bring to the boil then turn down and let simmer for 5 minutes so the pears soften a bit before baking. If using tinned pears ignore this step and just drain them well

Drain then let sit 5 minutes and drain again

Prepare the topping in the same way as the base - mix the flour, sugar and spices in a bowl then cut in the margarine. Stir in the oats then set aside

Sprinkle the well-drained pears with the sugar and cinnamon when the shortbread base comes out of the oven and spoon it over evenly with a slotted spoon to catch any more water

Spread the topping over the pears, press in lightly and bake for 35 minutes until golden

Let cool and set completely in the tin

Remove from the tin and slice into squares

These can be individually wrapped and frozen for up to a month