Heat the oil in a large frying pan over a medium-high heat. Season the lamb with salt and pepper and brown all over until just cooked, then remove to a plate and set aside.
Add the potatoes to the pan and pour in the stock so it just covers the potatoes; season with salt and pepper. Cover and cook for 10 minutes.
Remove the lid and add the butter, baby gem and peas. Fry in the buttery juices for 2 to 3 minutes, then add the vinegar and mustard. Return the lamb to the pan and cook for 1 to 2 minutes more to warm through. Serve sprinkled with the parsley.