Preheat oven to 350 degrees Fahrenheit. Heat a large oven safe pot over high heat.
Season the lamb shanks with salt. Add oil to the pan and brown the shanks on all sides. Once each of the shanks have been nicely browned, transfer them to a plate and set aside.
Meanwhile, puree the onion, carrot, celery, and garlic in a food processor. Add in tomato paste. Put this puree into the pan you used to brown the shanks and sauté for 3 to 5 minutes. Stir in the cranberry juice.
Return lamb shanks to mixture. Top with rosemary sprigs and place the lid on. Cook in the 350 degree oven for 2 1/2 to 3 hours, until lamb is fork tender. Remove lid during last 30 minutes of cooking to allow shanks to brown.
Season the shanks with salt as directed in step 2 of the recipe. Searing in oil in a skillet is an optional step. Searing your shanks helps the fat render and seal in the moisture so it’s worth the extra time and dishes. But we also recognize that one of the benefits of using a slow cooker is a short prep time so that you can throw everything in. Place the seared or un-seared meat directly into a slow cooker. Add in all remaining ingredients and cook on low 8 hours.
INSTANT POT/ELECTRIC PRESSURE COOKER:
Follow this recipe as directed, but instead of searing and baking the shanks in a large pot, you’ll do all of this in your pressure cooker. Sear the shanks as directed in step 2 using the sear setting on your electric pressure cooker. Saute the vegetable puree for 3-5 minutes then add in cranberry juice and the shanks. Cook on high pressure for 45 minutes, followed by a 15 minute natural release. Then, switch the release valve to the venting position. Remove lid once steam has stopped coming out.