Place the chopped onion in the slow cooker.
Mix the crushed clove of garlic with the oil, dried oregano, salt and black pepper.
Gently rub the seasoned oregano oil all over the lamb shanks.
Using a sharp knife, make incisions all over the lamb shanks.
Thinly slice the other clove of garlic and take the leaves off the rosemary (if using dried rosemary see tip below). Place a piece of garlic and push inside an incision and then push a piece of rosemary into another incision. Repeat until all the garlic and rosemary have been used.
Place the lamb in a large frying pan over a high heat and cook for 2-3 minutes each side until golden brown all over (this takes around 10 minutes). Don't move the lamb around too much when it's in the pan as the garlic and rosemary can come out.
Once golden, place the lamb shanks in the slow cooker with any bits of garlic and rosemary that are left in the frying pan.
Add the tomatoes, water or stock, and bay leaf and cover with the lid.
Cook on a low heat for 7 ¼ hours.
Sprinkle over the orzo/risoni, stir to make sure the pieces of pasta are coated in the tomato sauce then place the lid back on the slow cooker and cook for 35-45 minutes until the pasta is tender.
Remove the lamb shanks to a plate or board and seperate the meat from the fat and bones.
Return the lamb to the pasta and stir well to combine, then sprinkle over feta and serve straight away.