I don't usually use a spice rub with roast lamb, but I do use the following marinade for basting:
1/2 cup olive oil
1/4 - 1/2 cup lemon juice
2 tbsp. balsamic vinegar
2 tbsp. oregano
1 tbsp. chopped mint
1 tb onion powder
1 tbsp. chopped fresh thyme
1 tbsp. chopped Italian parsley
1 tbsp crushed rosemary
Salt and fresh ground pepper to taste
Place all ingredients into a sealed container (I use a Mason jar) and shake well. You can also increase the spices according to your own taste, or dilute the lemon juice slightly. You can also add curry powder for a more East Indian flair, or add cumin for more of a Middle-Eastern taste (
In addition, before cooking I cut small slits in the lamb and insert small slivers of fresh garlic. My Greek friend who taught me this recipe tends to use entire bulbs of garlic instead, but then, when we cook together we roast an entire lamb on a spit so we also make 8 batches of the above marinade as well.
As for the cooking method, I prefer to cook my lamb indirectly over a charcoal fire using a rotisserie. A gas grill with a rotisserie also works well with this recipe as well. Just make sure to baste often - my Greek friends and I use a spit over a couple of 4-foot-long charcoal troughs, so we baste it every 15 minutes or so. Your own results will vary, but 30 minutes may be OK. I tend to crank up the heat a bit at home and stick to the 15-minute basting schedule, but other opinions of this may vary, too.
I usually just use a meat thermometer and cook it to medium-rare or medium, depending on who's coming to dinner. Also, here's a meat temperature guide for lamb I got from About.com:
Medium-rare: 140° to 150°
Well done: 165° or a bit above
Anyway, I hope this helps. Enjoy the lamb and be sure to post some pics!