Build a fire or heat a gas grill to high. Place a large cast-iron pan or two-burner griddle over the heat and let it get very hot. Once hot, you can let the fire die back slightly. If using gas, reduce to medium once the pan is ripping hot.
Pound the pork to an even thickness of about ¾ inch.
Combine garlic, rosemary, and 6 tablespoons olive oil to make a rough paste. Season the pork aggressively on both sides with salt and pepper, then spread the garlic paste over both sides.
Brush the pan with the remaining 2 tablespoons olive oil. When it shimmers and is nearly smoking, lay the pork down. Cook without touching until a good crust forms, about 10 minutes.
While the pork cooks, surround it with the peaches cut-side down and dot with butter. Cook until soft and lightly charred, about 5 minutes. Move to a platter and tent with foil.
Flip the pork and cook in the remaining butter another 5–7 minutes. Rest under foil for 5 minutes, then slice and serve with the peaches.
Notes
The butterflied butt is thin enough to cook fast over high heat without drying out — the fat content carries it. Pounding to even thickness is the key move: hot, dry cast iron needs uniform contact or you get uneven crust. The peaches go in raw with butter rather than being cooked separately — they pick up the pork drippings and char just enough to concentrate sweetness without going to mush. Rosemary from the garden works perfectly here.