Preheat the oven to 200°C, gas mark 6, and grease a 30-cm ovenproof pan or baking dish.
Fry the leeks and garlic in the olive oil over a low heat for 5 minutes until softened.
Add the flour and cook for 2 minutes to form a paste. Add all of the milk and whisk until smooth. Stir over low heat until the white sauce comes to the boil.
Stir in the finely chopped beef biltong, mustard, seasoning and cheeses.
Layer the potatoes, cheese sauce and finely chopped moist biltong alternately in the dish finishing with extra cheese.
Bake in the preheated oven for 40-45 minutes or until cooked through and golden.
Garnish with the extra biltong and parsley before serving.