In a bowl mix salt, pepper and cumin together and season lamb shanks evenly on both sides. Heat a pot over medium heat, once hot add oil and cook shanks for 3 minutes each side, do in batches if needed, remove from pot. Add onions, shallots, celery, bell pepper cook for 2 minutes. Add garlic cook for 15 seconds. Then add tomato paste, rosemary and bay leaf. Add red wine, beef stock and shanks back to pot; bring to a boil cover with lid reduce heat to low and cook for 2 hours and 30 minutes, until shanks are fork tender. In a bowl mix flour and butter together, should resemble a paste. Remove shanks from pot, stir in flour and butter mixture and cook until mixture thickens. Remove, rosemary and bay leaves from pot. Place shanks back in pot.
Heat a pot over medium heat, once hot add butter and let butter begin to brown slightly, add thyme and cook for 15 seconds. Add onion and cook for 1 minute. Add remaining ingredients, bring to a boil and cook for 30 minutes or until celery root is tender. Add to a food blender and blitz on high until mixture is smooth. Taste and adjust seasoning.
To serve spoon celery puree in the center of plate, add shank and drizzle with sauce, garnish with parsley.