Fry your lardons in a medium sized skillet until nicely coloured and quite crisp. Set aside, reserving the oil. Add the butter to the hot pan then chuck in the potatoes. Add the onion also. Season well with salt and black pepper. Cook for 20 minutes or so on a relatively low heat, moving from time to time. You want everything to be a bit crispy, sweet and golden.
Add your lardons back to the pan and stir to combine. Create a well in the centre of your hash mix. Place the camembert square in the middle. Slash the 4 sides of the camembert with a sharp knife. Transfer the entire pan to the oven and bake at 180°C for 20 minutes, or until the cheese has become one with the hash.