Preheat the oven to 200 degress C. (390 Fahrenheit).
Peel the potatoes and give them a good wash.
In a saucepan, melt the butter over a medium heat, throw in the flour and mix quickly.
Add the milk and whisk well to avoid any lumps forming.
Stir until the sauce thickens slightly, then add the cheddar and parmesan cheese together with the double cream.
Peel and mince the garlic cloves, then add them to the sauce too.
Mix in the dijon mustard and season with nutmeg, salt and pepper.
Slice the potatoes thinly , then arrange one layer in a non-stick baking dish that has been greased with butter, so that each slice is overlapping.
Spread a quarter of the sauce over, then arrange another layer of potatoes.
Repeat with some more sauce, then another layer of potatoes and sauce again.
Cover the dish with foil, pop the dish in the oven and bake for an hour.
Remove the foil and bring the temperature to 220 degrees C ( 410 Fahrenheit) for the last 15 minutes, so the top can become golden.
Serve hot!