Preheat the oven to 180ºC/350ºF/gas 4. Start by making the potatoes: peel and finely slice the onion and garlic, then pick the thyme. Grate the cheese.
Melt the butter in a medium pan over a low heat, then add the onion and thyme and cook for 15 minutes, or until sweet and soft, stirring occasionally.
Meanwhile, in a separate pan, gently heat the milk and cream over a medium heat, seasoning with sea salt, black pepper, a good grating of nutmeg and the bay leaves.
Slice the potatoes by hand or on a mandolin (use the guard!) to around 3mm thick, dropping them into the milk as you go – it’s important not to soak the potatoes in water beforehand as you need the starch to thicken the cream. Cook the potatoes for 5 to 10 minutes, or until just softened, then remove and discard the bay.
When the onions are soft, add the garlic and cook for a few minutes, or until golden, then gently stir them into the potatoes, along with a handful of the grated cheese and the spinach.
Butter an ovenproof dish, then carefully tip in the potatoes, arranging the top layer neatly. Sprinkle on the remaining cheese, then bake in the oven for about 45 minutes, or until golden and crisp.
Once they’re done, take them out and keep warm, then turn up the oven to 220ºC/425ºF/gas 7.
For the lamb racks, peel and finely chop the garlic, pick and finely chop the herbs, then finely grate the lemon zest.
Season the lamb racks well with salt and pepper, then drizzle with oil. Heat a large frying pan on a high heat and once screaming hot, add the lamb and brown well on all sides for around 6 minutes – you may need to do one rack at a time. Transfer the lamb to a plate and set aside to cool slightly.
Using the back of a spoon, spread the mustard evenly over the seared fat. In a bowl, mix together the breadcrumbs, garlic, herbs and lemon zest with a little oil, then pat onto the mustard so the crumbs stick.
Transfer the racks to a baking tray and bake for 15 to 20 minutes for medium rare, or a little longer, if you prefer. Allow the lamb to sit for 5 to 10 minutes before carving into chops, then serve with the potatoes and seasonal greens.