If this is the first time I would:
1. Debone/butterfly it.
2. Use traditional lamby herbs like rosemary and garlic to marinate for several hours min.
3. Smoke for an hour 275/300 with some fruit wood like apple or cherry. Don't overdo the smoke.
4. Then grill both sides as required (5+ mins a side). You can sneak peeks at the meat with some small exploratory cuts if you are uncertain as to how cooked it is. A bit of charring is a good thing
5. Note 1: Lamb is very juicy so don't overcook thinking the juice is indicative of being undercooked.
6. Note 2: I have found that because lamb has so much fat, if smoked only, the meat has a greasy/oily texture which isn't attractive. With the grill finish, this goes and you can get some delightful mixes of flavours and textures.
This gives a good result reliably every time for me. The butterflying gives a more even thickness of meat for your initial goes at cooking - later you can leave the bone in which is more challenging.
I did one on the weekend where I also thinly coated in Chinese cha siu, which gave it a really nice sweet something, but this kinda covers the beautiful flavour of properly cooked lamb.
Cut into small pieces, cold leftovers are a great addition to salads. Oh, the taste of cold lamb fat on a school lunch sandwich - takes me back 50 years.
