Season the shanks with the salt and pepper. Place the onion pieces in a 4-quart slow cooker. Place the tied shanks on top of the onions. Add the Primal Kitchen Marinara, broth, and vinegar. Make sure that the liquid is three-quarters up the sides of the shanks. If needed, add more broth.
Cover and cook on low until the meat is fork-tender, about 6 hours. Gently remove the shanks and place them on a serving platter. Pour the liquid from the pot over the shanks and serve.
Store extras in an airtight container in the fridge for up to 6 days. To reheat, place a shank into a cast iron skillet with a cup of leftover sauce. Place in a 350 degree F oven for 5-10 minutes or until heated through.