Mix the almonds with all the other ingredients up to and including the salt, knead by hand to form a compact mass. Transfer the mixture to the prepared tin, smooth down with a spoon, pull upwards around the edges to form a rim of approx. 3 cm, cover and chill for approx. 30 mins.
Bring the cream, sugar and orange zest to the boil in a pan, simmer until the sugar has dissolved. Stir the gelatine into the hot liquid, then pour the mixture through a sieve into a measuring cup. Stir in the yoghurt, leave to cool. Spread the mixture over the tart base, cover and leave to set in the fridge for approx. 6 hrs.
Bring the reserved orange juice and sugar to the boil in a pan, simmer until the sugar has dissolved. Add the orange slices, cover and simmer over a low heat for approx. 15 mins. until the orange peel is soft. Remove the orange slices from the pan, reduce the liquid to a syrupy consistency over a very high heat for approx. 3 mins. (uncovered), leave to cool. Remove the tart from the tin, top with the orange slices and syrup.