Put a large pan of salted water on to boil. Scrub the potatoes and quarter or halve them so they’re all roughly the same size. Set aside. Pop the garden peas from their pods and set the peas aside in a bowl (see our tip on what to do with the pea pods).
The water in the pan should be boiling by now. Add the potatoes, half-cover the pan with a lid and bring it back to the boil. Turn the heat down a little and simmer for 15 mins till the potatoes are tender when pierced with a fork.
While the potatoes simmer, make the rest of the dish. Halve and peel the shallots, and finely slice them. Trim any ragged leaves off the lettuce and then slice the lettuce into 4 long wedges. Pick the tarragon leaves off their sprigs and roughly chop them. Crumble the stock cube into a heatproof jug and stir in 200ml boiling water to dissolve.
Put a frying pan or griddle pan on a high heat. Rub 1 tsp oil and some salt and pepper into each lamb leg steak. When the pan is hot, add the steaks and fry for 4-5 mins till browned underneath. Flip the steaks over and fry for another 4-5 mins (or add another 2-3 mins for more well done leg steaks).
While the lamb steaks fry, warm ½ tbsp olive oil in a separate deep frying an or wok, then add the shallots. Fry, stirring every so often, for 5 mins till the shallots are soft. Add the lettuce wedges to the pan, cut-side up, and pour the stock over the lettuce. Cover the pan and simmer for 5 mins.
Lift the lamb leg steaks out of the pan and pop them on a warm plate or board. Loosely cover with foil and set aside to rest.
The potatoes should be ready by now, so drain them and tip them back into the pan. Add the butter to the pan and toss to coat the spuds. Pop a lid on the pan to keep them warm.
Stir the peas and most of the tarragon into the pan with the lettuce. Simmer for 2-3 mins to just cook the peas.
Serve the lamb leg steaks with the buttery new potatoes, braised lettuce and peas, garnished with the last of the tarragon.