Loosen the skin of the spatchcock by sliding fingers between skin and meat at neck joint, divide butter evenly between spatchcocks, and spread as evenly as possible under the skin. (don't worry if it is a little lumpy, it melts once in the oven which helps it to disperse. I know this step can be a little tricky since I have made this several times, but it is worth it.).
Place 2 thyme sprigs inside the cavity and tie legs together with kitchen string. Wrap a slice of prosciutto around each spatchcock and place in a baking dish.
Drizzle combined oil and juice over spatchcocks and roast in a preheated oven 180c for 45-50 Min's or until cooked through.
Serve spatchcocks with bacon and sage sauce.
For the Sauce.
Heat oil in pan, add onion, garlic and bacon. Cook, stirring until onion is soft and bacon lightly browned.
Add water, stock, wine and mustard.
Bring to boil, simmer, uncovered for 5 minutes or until reduced to 1 cup.