Put the potatoes (large ones halved) in a pan of cold water, bring to the boil and then simmer for 10-15 minutes until tender. Drain, cut into halves and keep warm.
Meanwhile, mix ½ tablespoon of the oil with the lemon zest and brush over the chicken. Heat a griddle pan to medium and cook the chicken for 5-10 minutes on each side, until cooked through and no pink remains.
Heat the remaining oil in another pan and quickly fry the cut sides of the lettuces. Remove and set aside. Fry the shallots for 3 minutes, then add the peas, mint and stock. Simmer for 2 minutes, then add the crème fraîche and the lettuces. Simmer for a further 2 minutes.
Slice the chicken and serve on top of the peas and lettuce, with the potatoes sprinkled with the extra mint on the side.