Whisk the egg whites in a clean bowl with a hand-held electric whisk until peaking. Gradually whisk in the sugar, a spoonful at a time, until thick and glossy. Add the ground almonds and stir in until combined.
Drop dessertspoonfuls of the mixture, slightly apart, on a large baking sheet lined with baking parchment. Press an almond on top of each.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for about 15 minutes until the biscuits are pale golden and just crisp. Leave on the paper for 5 minutes, then transfer to a wire rack to cool.