She's barely old enough to be called mutton - more like overgrown hogget

At that age, as long as she's not too fat, her meat will not be tough at all. She could be cooked by a long slow roasting in a covered roaster tin, then served like any other joint once the meat's falling off the bone. Even chops should be fine, and if they were a bit tougher than you like then do them in a casserole with herby gravy.
The only real mutton I have used was a handful of ancients (at least 10 yo) and I had them minced and I made them into very tasty burger shaped things with strong flavours added such as chilli, herbs, N African spices, lots of black pepper. We don't use these as actual burgers in a bun, but grill them then serve with a variety of sauces - great for a quick meal.
There's also sausages which come out a bit harder than pork, but made with those same spices are delicious.