Preheat the oven to 200°C/fan180°C/gas 6. On a lightly floured surface, roll out the pastry to fit the base and sides of a 30cm x 20cm baking tray. Spread the jam over the pastry base.
Place the butter and caster sugar in a large bowl and beat with an electric hand whisk until smooth and creamy. Beat in the eggs, 1 at a time, adding 1 tbsp ground almonds after each addition. Add the flour, remaining almonds and almond extract and stir well. Spoon the mixture over the jam, spreading to make an even layer.
Bake for 35-40 minutes until well risen, firm and golden. Remove from the oven and leave to cool in the tin.
In a small bowl, mix together the icing sugar and 3 tbsp water to make a fairly thick icing. When the tart is cold, spoon the icing on top, spreading evenly. Decorate with glacé cherries and flaked almonds and leave to set. Cut the tart into squares or rectangles and arrange on platters to serve