Preheat the oven to 200°C/400°F/gas 6. Peel and halve the onion. Wash and finely slice the potatoes, then very finely slice with the onion using patient knife skills, on a mandolin (use the guard!) or in a food processor. Tip the potatoes and onion into a large sturdy roasting tray and season with a pinch of sea salt and black pepper. Grate over a few scrapings of nutmeg and crush in the unpeeled garlic cloves and pour in the single cream. Tear in the anchovies (if using) and finely grate over most of the Parmesan. Add the bay leaves, pick in the herbs and add 2 tablespoons of olive oil. Use your clean hands to mix and toss everything together, then put the tray over a medium heat. Pour in 300ml of boiling kettle water, cover tightly with tin foil and leave on the heat for 5 minutes. Remove the tin foil. Finely grate over a layer of Parmesan. Drizzle the remaining thyme sprigs with oil, scatter on top and put into the oven on the top shelf to cook for 30 minutes, or until golden brown and bubbling.