Heat olive oil in a frying pan over medium-high heat. Season lamb shanks generously with salt and pepper. Brown on all sides (8-10 minutes total). Transfer to slow cooker.
In the same pan, sauté onions, carrots, and celery for 5-6 minutes. Add garlic and tomato paste, cook 1 minute more.
Pour in red wine, scraping up any browned bits. Bubble for 2 minutes, then add lamb stock. Bring to a simmer.
Pour everything over the lamb shanks in the slow cooker. Add bay leaves, thyme, and rosemary. The liquid should come halfway up the shanks.
Slow cook on low for 7-8 hours or high for 4-5 hours. The meat will be practically falling off the bone.
Carefully remove shanks and put to one side. Keep warm.
Strain cooking liquid through a sieve, pressing vegetables to extract flavour. Skim excess oil and then pour into a pan and simmer until reduced, or whisk in cornflour/cornstarch slurry, made up of 3tsps of cornflour and 3tsps of water, and simmer until thickened.
Plate the lamb shanks and serve with mashed potatoes and vegetables. Pour over lashings of the gravy. And garnish with fresh herbs.