Preheat a char-grill or barbecue plate on medium high heat.

Combine garlic, sage, chilli and salt into a small food processor or mortar and process until broken down and smooth. Add softened butter and oil and mix well; set aside.

Place the bird breast side down on the cutting surface. With a sharp pair of poultry shears, cut along one side of the bird"s backbone, and then the other. Lift out the backbone, and throw out, or save for the stockpot. Spread the bird open and locate the breast bone. Loosen and lift it out with your thumbs. Using large metal skewers, push two skewers through the thickest part of the bird and brush liberally with 3/4 of the garlic butter mixture and season with salt and pepper.

Grill, skin side down 10 minutes, turn over and cook a further 10 minutes. Remove and place onto a tray and cook in a moderate oven for a further 15 minutes.

Meanwhile, place the remaining garlic butter mixture into a large deep frying pan and add potato slices and prosciutto. Cook, turning potatoes over until golden and prosciutto is crisp. Season with salt and freshly ground black pepper.

Remove skewers from spatchcock and cut in half; served with potato and proscuitto.