Warm 2 tsp olive oil in a pan. Add the diced lamb. Cook over a medium-high heat for 5-8 mins till the lamb is browned all over. Stir every now and then. Drain off any excess liquid.
While the lamb browns, peel and finely chop the onion. Peel and grate or crush the garlic. Roughly chop the tomatoes. Fill a kettle and boil it.
Add the onion to the lamb. Season with salt and pepper. Fry for 5 mins till the onion looks glossy and has picked up a little colour. Stir every now and then.
Stir the garlic into the onion and lamb. Cook and stir for 1 min. Add the tomatoes to the pan with the cinnamon stick and 2 sprigs of the rosemary. Add 600ml hot water from the kettle. Pop on a lid.
Bring the lamb stew to the boil. Turn the heat down and simmer for 40 mins till the lamb is just tender and cooked through. Fish out the cinnamon stick and rosemary sprigs.
Drain the butter beans. Rinse them. Add them to the pan. Pop the lid back on the pan. Simmer for another 10 mins to heat everything through.
Roughly chop the chervil. Crumble half the pack of feta. Taste the stew and adjust the seasoning if you think it needs it. Ladle it into warmed bowls. Top with the crumbled feta and chervil to serve.