Peel and quarter the onion then seperate each segment into layers and place in the bottom of the slow cooker.
Slice the potatoes. If using large potatoes, cut into quarters or sixths, if using small or baby potatoes slice in half or leave whole. No need to peel them. Place into the slow cooker with the onion.
Slice 2 of the cloves of garlic and break bits of rosemary off the sprigs.
Using a sharp knife, make incisions all over the lamb shoulder or lamb leg, slicing deeply but not all the way through the meat.
Stuff each hole with a slice of fresh garlic or small sprig of rosemary, until all the rosemary and garlic are used.
In a small bowl mix the oil with the crushed garlic, dried oregano, lemon juice and zest, and salt and pepper.
Rub this marinade all over the lamb.
Place the lamb in a hot non stick pan over a medium heat and sear all over until golden brown. You don't want the pan to be super hot otherwise it will burn the rosemary and garlic and become bitter.
Remove the pan from the heat and place the seared lamb on top of the potatoes and onions. Pour the stock into the frying pan and use a non stick spatula to loosen all those bits of toasted garlic, herbs and lamb that are stuck to the pan.
Pour this over the lamb, potatoes and onions and add the bay leaf.
Cover with the slow cooker lid and slow cook for 8 hours, until the lamb and potatoes are cooked and tender.
Serve straight away.
If there is a lot of thin gravy left in the slow cooker, use a ladle to scoop it out and place it in a saucepan. Simmer to reduce and thicken.