Step 1: Heat the oven to 200°C.
Grease a 20cm by 25cm dish. To make the choux pastry, place the water and butter in a medium saucepan and bring to the boil.
Remove from the heat and stir until all the butter has melted.
Add the flour and rapidly beat until the mixture is smooth, then return to the heat and beat constantly until the mixture falls away from the side of the pan.
Remove from the heat and allow to cool for 15 minutes.
Step 2: Add a little beaten egg and mix well until shiny and smooth.
Repeat until the mixture has reached a dropping consistency.
Now add half of the grated cheese and mix.
Step 3: Spoon the choux pastry mix around the edge of the prepared baking dish, place in the oven and bake for 30 minutes or until puffed up and browned.
Reduce the heat to 150°C.
Step 4: To make the filling, heat the olive oil and butter in a pan.
Add leeks, fry for 3 minutes, then add the garlic and cook for another minute.
Remove the pan from the heat and mix in the flour.
Return to a low heat, add a little stock and mix until smooth. Add wine, pepper and mustard and mix well, bringing to the boil for 10 seconds, stirring constantly so that the sauce does not stick to the bottom.
Add the ham, then spoon the mixture into the dish and sprinkle the remaining cheese on top.
Return the dish to the oven and bake for another 25 minutes or until bubbling and golden.