Peel and dice the sweet potato and then simmer for around 15 minutes or until cooked. Set aside to cool.
Purée the sweet potato either using a food processor or hand blender and then place in a large bowl.
Separate the egg yolk from the egg white.
Add the ground almonds, sugar, egg yolk, lemon zest and honey and mix well either by hand or using an electric mixer.
Take a teaspoon of the mixture, form a biscuit shape by hand and place on a lined baking sheet. Repeat with the rest of the mixture.
Brush the egg white over the tops of the cookies and sprinkle a few chopped pistachios on each cookie.
Bake for around 10 minutes or until starting to turn golden at the edges. Turn the tray around after about 5 minutes so that they cook evenly. Depending on the size of your oven and size of your baking sheet you may need to bake in two or three batches.