First of all you need a leg of lamb. A good sized leg of 2,5 kg serves 6-8 people.
A lot of people are afraid to go ahead and cook such large quantities of meat, thinking it's too difficult. Well, it shouldn't be.
The trick is to use a cooking thermometer.

Ingredients

Serves 6 - 8:

1 leg of lamb 2-2,5 kg

1-1,5 kg potatos

1-2 onions

1-2 green bellpeppers

Garlic, as much as you like.

Rosemary, fresh or dried.

Oliveoil

Salt and pepper.

Preparing

Turn on the oven. Set the temperature to 150° - 170°C.

Dry off the leg and rub it in with salt and pepper. Sometimes there's a quality mark stamped on by the butcher. Slice this off¹ with a sharp knife.

With the same sharp knife, cut small pockets and stick in thin slivers of garlic and small sprigs of rosemary.
How much depends on how much you love garlic. Let's say 1 - 4 cloves. If you don't have fresh rosemary, you can use dried.
Crush it a bit in a mortar and rub it in. There's usually so much fat in the meat that you don't need to add any. A light brush of oliveoil if you insist..

Now it's time to stick in the thermometer. Stick it in the thickest part of the thigh, and make sure it doesn't hit the bone.

Vegetables

Wash and slice the potatos (enough to aforementioned 6 - 8 people) in lengthwise slices², Do the same with a couple of onions and green bellpeppers. Put the vegetables in the dish around the lambs leg. Brush lightly with olive oil. Add salt and pepper. And why not throw in a handful of garlic cloves, whole, just peeled. They will bake nicely along with the rest, adding a lovely nutty taste.

Now we're cooking!

Put the dish near the bottom of the oven. The cooking will take 2 - 2,5 hours, depending how your oven works.
The meat is nice and pink when the thermometer shows 70°C. Pay attention the last half hour because at 76°C it's really done!

Serve with green salad and lots and lots of good red wine.