Preheat the grill and brush with a metal grill brush to loosen any crud and soot. After the grill has been brushed, run an oiled towel over the grill to pick up any loosened crud.
In a large bowl, toss the potatoes and onions together with the rosemary, garlic, crushed red pepper, salt, to taste, and a little olive oil. Try to keep the onions together in rounds, they will fall apart while grilling but try and keep them together as long as possible; it just makes things easier.
Arrange the potatoes and onions on the preheated grill. Once the potatoes have developed lovely grill marks, rotate them a quarter turn to create a lovely crosshatch pattern, (dont worry about the onions, they fall apart and you wont see the marks). Turn the potatoes over and repeat the process, turn the onions over to cook the other side as well. Cook the potatoes and onions until they are soft all the way through, about 8 to 10 minutes on each side.
Remove from the grill and serve hot or at room temperature.