It uses shoulder which I think is the sweetest cut of lamb and is usually less expensive than the leg. I use boned and rolled shoulder, which has most of the fat removed however you can use an unboned joint but I suggest you trim as much fat as you can. The cooking time will be about the same.
Most unboned shoulders weigh about 1.2 to 1.6kgs (2.5 to 3.5lbs) which is plenty for 4 people with perhaps leftovers for salads or sandwiches. A larger shoulder should feed 6; just increase the cooking time by about 15 minutes. Adding more beans will make this dish go further.
With the beans I do not think you need potatoes or any other starchy vegetable. Just some good greens - keep it simple with broccoli or cabbage.
The beans used in France are haricot blanc which go under various names in various places - basically it is the bean used in baked beans. You can use canned beans if you wish, or are in a hurry, but if you have the time cook white beans - either fresh or dried. Fresh, shelled beans, if you are lucky enough to get them, should be rinsed and put in cold water or stock, brought to the boil and simmered for about 30 minutes, until they are tender. Try not to over cook them as this causes them to crumble. Dried ones are cooked as per directions on the packet. Be aware it is likely they will need to be soaked overnight. I reckon one and a half cups of dried beans is plenty as they increase a little in size when soaking.
Shoulder of lamb on white beans & thyme
One boned, and rolled shoulder of lamb 1.2 n- 1.6 kgs (2.5-3.5lbs)
Two cups of cooked beans ( about 400-500 grams)
1 tbspn of olive oil
1 medium onion- sliced and chopped
1 clove of garlic
1 tspn of dried thyme or 2-3 tspns of fresh thyme
200 mils of chicken stock
About an hour and a half before serving:
Heat oven to 180C, 375F, gas mark 5
Warm your stock.
Peel & chop the onion into a small dice.
Peel and slice the garlic while the oil heats in a large frying pan over a medium heat.
When the oil is hot add the onion & garlic and push them around the pan with a wooden spoon. As they soften add the thyme.
Stir the beans into the pan.
Place this mixture into a shallow casserole dish and pour the warm stock over it.
Pop the lamb on the top of the beans. The whole thing goes into the oven and stays there for about an hour. This should give you nicely pinkish lamb.
Check after about 40 minutes to make sure the stock has not all been cooked off. If so add a small amount, a cup or so, of boiling stock or water.
When the lamb is cooked remove from the oven, cover with a clean tea towel or foil and leave to stand for 10 minutes before cutting into slices. Be sure to remove any string that maybe holding the lamb together before you starting slicing it up, it makes it much easier.
I put a serving of beans onto the plate and position a slice of lamb on top.