Does lamb get more tender the longer you cook it?

Definitely, but it's also important how you cook it. Lamb is naturally tougher meat in general, so to tenderize it, you'll want to either stew the meat for a long period of time or slow-roast it until the muscle fibers relax into a state of juice, delicious tenderness. A simple way to roast lamb is to season it well and place it in a baking dish. Slide the lamb into a preheated oven at 325 °F (163 °C) for 1 hour. Then, rotate the lamb and let it bake for another 30 minutes. After that, check to see how tender the meat is. If it's still tough and doesn't come apart easily, bake it for another 30 minutes. Be patient and give the lamb the time it needs to tenderize.