Wash peaches, cut in half vertically and remove pit. Lay peach halves cut side up in a cast iron skillet, trimming bottom slightly if needed to make peaches sit flat. Scoop 3tbsp (lightly packed) crumb mixture onto each peach half. Press down mixture until flat and covering entire peach half. Place in centre of grill using indirect heat or alternatively place in oven and cook for 30-40 until peaches are tender and crumble is golden and crisp.