Season the lamb shanks with salt and pepper. Heat olive oil in a large skillet over medium-high heat and brown the lamb shanks on all sides. Transfer to the slow cooker.
In the same skillet, add onions, garlic, carrots, and celery and sauté until softened, about 5-7 minutes. Add the red wine, scraping up any browned bits from the pan. Bring to a simmer and cook until reduced by half.
Pour the vegetable and wine mixture over the lamb shanks in the slow cooker. Add beef broth, diced tomatoes, rosemary, thyme, and bay leaves.
Cover and cook on low for 8 hours or until the meat is tender and falls off the bone.
Remove bay leaves before serving. Serve the Lamb shanks with the rich herbed gravy.