Preheat the oven to 160˚C/gas mark 3.
Season the lamb with salt and pepper and fry it in a tablespoon of vegetable oil over a medium heat, until evenly browned. Don’t rush it — the flavour and colour is much better when this is done gradually.
Place the lamb in the oven for 8–10 minutes (if you haven't used a frying pan with a metal handle, transfer the lamb to a foil-lined baking tray instead). Then, remove from the oven and frying pan and leave to rest for 10–15 minutes.
In another frying pan (or the same frying pan that you used for the lamb, if you didn't use one with a metal handle), heat the remaining vegetable oil over a medium heat and cook the bacon until crisp and golden, for 10-12 minutes.
Add the onions and cook for a further two minutes.
Cover the bacon and onion with lamb stock and bring to the boil.
Add the peas, butter, lettuce and mint, season with salt and pepper and remove from the heat.
To serve, slice the lamb, pour the pea and bacon mixture into a large bowl or individual servings, and arrange the lamb slices on top.