Photograph: Jan Baldwin
To make the salsa verde, drop the parsley into a pan of boiling water. After 30 seconds, drain and run under cold water. Dry and finely chop the parsley. Tip into a bowl and mix in the remaining salsa ingredients and season.
Preheat the oven to 180°C, fan 160°C, gas 4. Bake the potatoes for 1 hour 15 minutes or until tender. Halve and scoop out the potato and discard the skins. Put the potato through a mouli or mash well in a bowl. Heat the milk and butter and incorporate into the mash. Season and keep warm. Increase the oven temperature to 220°C, fan 200°C, gas 7.
In a hot pan, sear the lamb racks in a little oil until coloured all over. Transfer to a roasting tin and roast in the oven for 8 minutes until cooked to medium rare, 15 minutes for medium or 20-25 for well cooked. Leave the racks to rest out of the oven for 10 minutes.
Meanwhile, heat a griddle pan, cook the lettuce until coloured and tender but still with a bit of bite. Dress with the rapeseed oil and season. Slice the lamb racks and serve with the mash, lettuce and salsa verde.
Get ahead: make the salsa verde a few hours ahead