Squish down on the garlic cloves with the flat side of a knife to bruise them and put them into a large freezer bag. Add the peppercorns, pressing down on them from outside the bag with the end of a rolling pin or a tin of soup to bruise them. Now add the steak, onion quarters, olive oil, vinegar and Worcestershire sauce and mix to combine. Seal the bag and place it in a dish to prevent leakage. Refrigerate for 12 or so hours, or marinate at room temperature for up to 2 hours. The marinating step is critical, so don't cut it short.
Bring the steak to room temperature before cooking on a griddle for about 6 to 7 minutes a side; it's hard to say exactly how long it'll take as it depends on how you like your meat. I like mine pretty well still quaking and trembling on the plate, so be prepared to cook it for longer if you want better-done meat. Test by prodding before you start poking knives in: if it's very very soft and bouncy, it's blue; springy, rare; springy but with resistance, medium-rare to medium depending on that resistance; hard, well, you know the answer to that.
Take the steak out and lay it on a large piece of foil, sprinkle with salt, and then wrap the whole parcel in another piece of foil and let it stand for 10 minutes before carving thinly, crosswise (on the diagonal). Arrange on a plate lined with arugula leaves. Put a plate of lemon quarters on the table too, for squeezing over the meat as you eat. Serve with potatoes on the side.
Preheat the oven to 400 degrees F.
Wash and dry the potatoes, but don't bother to peel them, and cut them into about 3/4-inch dice. Toss with the oil on a large baking sheet, using your hands to mix well. Tuck in some torn up rosemary sprigs, and roast for about an hour, turning once or twice during that time, until they wonderfully crispy and golden but still soft on the inside. When they're done, remove to a large plate and sprinkle with salt.