Pre-heat your oven to 180C / 356F / gas mark 4.
In a sauce plan, melt the butter in the olive oil on a low heat before adding the sliced leek and the salt then cook for 2-3 minutes.
Add the diced carrot and cook for another 5-8 minutes until the carrots and leeks have started to soften.
Add the mushrooms and crushed garlic to the pan and cook for another 5 minutes.
Next, add the plain flour to the ban and stir until the butter and juices from the vegetables have created a paste with the flour.
Pour in the milk and mix well so that the flour dissolves in the milk creating a thick sauce.
Let the sauce simmer gently before adding the cheese, mustard and black pepper.
Once the cheese has melted add in the frozen peas.
Cook until the peas have thawed and the cheese sauce has thickened.
While the cheesey vegetables are cooking, roll out your pastry and cut out a lid, exactly the same size as the top of your oven dish.
Cut out "say cheese" using alphabet cutters from the leftover puff pastry.
Spoon the cheesey vegetable mix into your oven dish.
Place the pastry lid on top of the vegetables and press the edges down using a fork.
Brush the pastry with the beaten egg and stick the "say cheese" letters on top.
Place into the centre of your oven and cook for 45 minutes or until the pastry is crisp and golden brown.
Serve immediately with some steamed green veggies.