Bruise the rosemary needles by rubbing the sprigs in your hand, and strip the needles into a bowl. Add the garlic, olive oil and pepper and then the meat. Toss all together and leave for at least two hours (4-6 hours, even better).
Remove the meat from the marinade and wipe it lightly (you don't want it dripping with oil), but leave the bits of garlic and rosemary sticking to it. Place the portions over a fairly hot, but not flaming grill. Let the meat sear on the bars for a couple of minutes and they should turn without sticking, then turn them every couple of minutes to prevent too much blackening on one side. Butterflied pieces should take between 9-15 minutes.
Finely chop the garlic on a large chopping board. Then add the herbs, anchovies, and capers and chop all together until the ingredients are well mixed and fine in texture. Transfer to a bowl and mix in the mustard, sugar, lemon juice and black pepper, plus enough olive oil to give a glossy, spoonable consistency. As you add the last ingredients, taste and tweak the mixture until you get something with the balance you like.