Beat the sugar, butter and lemon zest together until the texture is light and fluffy.
Add the lemon juice, egg and egg white as you continue to beat. The mixture should be smooth in texture.
In a second bowl, mix together the flour, ground almonds, baking powder and salt.
Spoon half the dry mixture into the butter mixture, and beat together.
Add the remaining dry ingredients, and continue to mix into a stiff cookie dough.
When the mixture is smooth, cover the bowl, and refrigerate for about 30 minutes.
When the dough is ready, preheat the oven to 350 ºF, and line a baking sheet (or two) with parchment paper.
Spread the almond slices on a plate in an even single layer.
Take the dough by the tablespoonful, and roll each spoonful into a ball.
Roll each ball in the almond slices, and place on the cookie sheet.
Bake for about 11 to 12 minutes. They should be only just set, and not overcooked. You want a nicely soft center.
Let the cookies cool on a wire rack for at least 30 minutes before eating.
You can store them in the fridge in an airtight container for up to a week.