Boil the potatoes until soft when pierced with a fork.
Before the end of the cooking time add the green beans and cook for 2-3 minutes.
When they have cooled enough to handle, slice them thinly and set aside.
Melt the butter until it is foaming in a medium sizes pan, add in the flour and whisk until it becomes creamy.
Cook for 2 minutes, stirring a little before whisking in the milk.
Continue to whisk and cook until the sauce has thickened.
Season with salt and pepper then stir in the nutmeg.
Remove from the heat and stir through the pesto sauce.
Preheat the oven to 200°C / 400°F / Gas mark 6.
Spread a little of the pesto sauce across the base of medium sized roasting tin.
Layer sheets of the pasta across the base then top with the sauce, green beans, slices of potatoes and a generous sprinkle of the cheese.
Repeat the process with the remaining ingredients until the final layer of pasta has been placed.
Pour over the remaining sauce, sprinkle with Parmesan and dot with a little butter.
Bake for 25 minutes until bubbling and golden.
Serve with a fresh salad.