Preparing the brussel sprouts. Working over a big bowl with a sharp paring knife, cut off (and discard) a 1/4-inch or so of the base of each sprout, freeing the outer leaves. Now stick the point of the knife into what's left of the base and slice out the tiny core, in one cone-shaped piece - just as you would cut out the bigger core of a cabbage or cauliflower. This loosens the inner leaves. Discard the small core.
Begin peeling off the outermost leaves: discard only wilted or blemished leaves; drop all the fresh dark green leaves, even thick ones, into the bowl. Keep peeling off the leaves until you reach the tiny ones that can't be pulled apart. Cut this bundle in slivers and drop them into the bowl. When all are done you will have a large fluffy pile of leaves.
Put the oil and the garlic in the skillet and set over medium heat.(Now I quite often grate the garlic over the sprouts just before I add the vinegar so that it has a fresher flavour) Let the garlic cook and caramelize lightly for 4 minutes or so, shaking the pan now and then. Dump in the sprout leaves, shake the pan to spread them out, and then sprinkle the salt all over.
Add the stock/water. Cover the skillet and let the leaves cook and wilt for 4 to 5 minutes, giving the pan an occasional shake, then uncover and turn them well with a big spoon or tongs. The leaves should be sizzling but not browning - lower the heat if necessary. Cover again. Cook another 4 to 5 minutes until the leaves are soft, greatly reduced in volume but still green and glistening.If all the liquid evaporates, before the sprouts are cooked. add more about 1/4 at a time. Sprinkle over the vinegar and black pepper, stir. Serve the Brussels sprouts hot right from the frypan or turn them onto a warm platter.