For the pastry: Pulse together the flour mix, icing sugar, xanthan gum, and salt in a food processor a few times. Add the butter and pulse until the mixture resembles rough breadcrumbs.
Add the egg and vanilla extract, pulsing until a rough dough forms around the blades; if needed, add iced water, 1 tbs at a time, pulsing between additions until the dough forms.
Turn out, gather, and pat down into a disc. Wrap in clingfilm and chill for at least 1 hour.
When ready to use, preheat the oven to 180°C (160° fan). Roll out the dough on a lightly floured surface into a large round, approximately 1/2 cm (1/4 inch) thick.
Use the pastry to line the base and sides of a 20 cm (8 inch) pie or flan dish. Cut away any excess overhanging pastry. Prick the base all over with a fork.
Line the pastry with greaseproof paper and fill with baking beans. Blind-bake until golden and dried out, about 20 minutes. Remove from the oven and discard the beans and paper.
Return to the oven for 5 minutes to dry out the base. Remove to a wire rack until ready to use.
For the filling: Combine the eggs, milk, condensed milk, sugar, vanilla extract, and salt in a clean food processor. Process until evenly combined and smooth.
Arrange the fruit in the pastry before carefully ladling or pouring in the filling.
Bake for 40-55 minutes until the filling is set and golden-brown on top.
For the biscuits: In the meantime, gently cream together the butter, sugar, and vanilla extract in a mixing bowl with an electric whisk until pale, about 2 minutes.
Stir together the flour mix, xanthan gum, and salt in a separate bowl. Stir into the creamed butter in three additions until a soft dough forms.
Turn out dough onto a lightly floured surface and gently shape into a disc. Wrap in clingfilm, and chill for 30 minutes.
After chilling, divide the dough in two and shape into logs, about 4 cm (1 ½ inch) in diameter. Wrap in sheets of greaseproof paper, twisting the ends in opposite directions to secure. Chill for a further 15 minutes.
When ready, remove the flan from the oven to a wire rack to cool. Increase the oven to 190°C (170° fan) and line a large baking tray with greaseproof paper.
Cut across the dough into 1 cm (1/2 inch) thick rounds. Arrange on the baking tray, spaced apart, and brush with the beaten egg yolk mixture.
Bake until golden and set about 10-12 minutes. Remove from the oven and let the biscuits cool on the tray for 10 minutes before transferring to a wire rack to finish cooling.
When ready, serve alongside the flan; if desired, caramelise the top of the flan with a chef’s blowtorch before serving.
If desired, the flan can be cooled and chilled overnight to be served cold the next day.
Serves 8 or 24 biscuits approx.