You will need three 20–25 cm loose-bottomed flan tins and one or two baking sheets.
Put the sugar, almonds and flour into a food processor and turn on at full speed for a few seconds. Add the butter dice and work again until just blended in. The mixture will resemble fine breadcrumbs. Add the egg and yolks, the lemon zest, rum and a minute pinch of salt, and work again until the pastry forms into a ball.
Scrape this out on to a sheet of clingfilm and roll up the film to form the dough into a cylinder with a diameter of 5 cm. Chill for at least 2 hours.
The dough is impossible to roll out, so cut thin discs off the end of the cylinder and overlap them slightly to cover the bottom and sides of each tart tin, pushing down with your fingers to make as even a pastry shell as possible. It should be slightly more solid round the edges and pushed right up to the top as it will shrink slightly as it bakes. Be careful to press into the corners so there is no air between the tin and the pastry.
An alternative method of rolling out the dough is to do it between two sheets of clingfilm, peeling one layer off before you lift it into the flan ring and the second when you have pressed the pastry into the ring.
Cover with clingfilm, pressing this well into the pastry case, and freeze until needed.