For the braised shallots, preheat the oven to 180C/350F/Gas 4.
Place the shallots onto a large piece of aluminium foil, drizzle with olive oil and place the thyme on top.
Fold the foil into a parcel, making sure that the edges are sealed, and place onto a baking sheet. Bake in the oven for 40-50 minutes, or until the shallots are soft. Remove from the oven and set aside.
For the lamb, preheat the oven to 200C/400F/Gas 6.
Heat the oil in a frying pan until hot. Season the lamb with salt and freshly ground black pepper and fry for 2-3 minutes on all sides, or until golden-brown all over.
Place the remaining lamb ingredients into a baking tray and sit the lamb loin on top, skin-side up. roast in the oven for 30-35 minutes (for medium), or until cooked to your liking. Remove from the oven and set aside to rest for 5-10 minutes. Remove the garlic cloves and set aside, discard the remaining vegetables.
For the herb purée, blanch the parsley in a pan of boiling water, drain and plunge into cold water. Drain again and set aside.
Heat the butter in a frying pan and fry the shallots for 2-3 minutes, or until softened. Stir in the spinach, sorrel and chervil and cook for 2-3 minutes, or until wilted.
Bring the cream to the boil in a small saucepan, stir in the parsley, spinach mixture and garlic. Using a stick blender, blend the mixture to a purée. Season, to taste, with salt and freshly ground black pepper.
To serve, heat a little olive oil in a frying pan and fry the braised shallots for 2-3 minutes, or until golden-brown. Carve the lamb loin.
Smear a spoonful of herb purée onto each of 4 serving plates and place the lamb loin on top, at an angle. Spoon the shallots alongside and place the reserved garlic cloves around the edge of the plates. Drizzle any pan juices over the lamb.