Preheat the oven to 200°C/fan 180°C/gas mark 6. Using a long, thin knife, make small cuts all over the lamb rumps and push in your finger to turn the cuts into small holes. Then push the frozen anchovies, followed by the bacon lardons, rosemary and thyme sprigs into the various holes you’ve made, and season the lamb with salt and pepper.
Heat the olive oil in an ovenproof frying pan over a high heat. Place the lamb rumps in the pan, skin side down, and cook for 5 minutes, turning them to make sure they brown well all over. Then place the pan in the preheated oven for 8–10 minutes. Remove the lamb rumps from the oven and let them rest for 10 minutes. This will leave them perfectly pink in the middle.
Meanwhile place the stock, saffron and balsamic vinegar in a medium-sized saucepan and boil until reduced by a third.
Pour the meat juices from the lamb rumps into the reduced sauce. Gradually stir in 30g of the butter until it melts and the sauce thickens.
Heat a large sauté pan, put in the rest of the butter and, once hot and melted, throw in the spinach and cook over a medium heat until just wilted. This will take 2–3 minutes.
To serve, place a mound of spinach in the centre of each plate. Carve the lamb rump into slices, place them on top of the spinach and pour over the sauce.