Mustard, rosemary, garlic, salt, pepper and brown sugar. Mix them all together.
Remove the netting from the leg of lamb and use a good sharp knife to make sure it fully opens and is more or less an even thickness. You’re doing that because the next step is to slather the above mixture all over the inside - the even thickness makes sure the flavor gets into the large muscles.
Put the netting back on and slather the outside.
Now at this point, your best bet is to throw this guy on the grill, sear the outsides nice and dark and then put it off the direct heat until it gets to 135-140 (depending on your preference). This takes about 75 minutes for me, but I trust to the thermometer, not the clock.
You can do the same recipe in the oven, but the char and smoke are a key element of the flavor for me. Still, you’ll get something pretty edible even in the oven.
(Note: I think this recipe is based on Alton Brown’s, but it’s in my head now and I don’t know how close mine is to the original. But if you need measurements other than “enough to look right” you’ll probably want to look AB’s version up.)
(Note 2: Mint is never allowed anywhere near my lamb. I don’t know where that tradition started, but I can’t stand that flavor combination.)