Preheat your oven to 160F/180C. In a large oven tray, put all your prepared vegetables, garlic and herbs then generously glug over your olive oil and stir to coat. Roast in the oven for 1 – 1 1/2 hours, stirring occasionally.
Meanwhile slice your potatoes thinly and set aside. Make your cheese sauce: melt the butter in a small pan, take off the heat and stir in your flour. Return to the heat and gradually add your milk stirring all the while until you have a nice thick sauce. Season. Take off the heat and stir in most of your grated cheese and a handful of grated parmesan.
In a large oven dish, spoon in your roasted vegetables in an even layer. Top with the potato slices, slightly overlapping to cover the vegetables. Then pour over the sauce and smooth over the top. Scatter with the remaining cheeses and bake for 40-45 minutes until nicely browned and the potatoes are tender. Serve.