Heat 1 tbsp oil in a casserole dish and brown the lamb shanks well all over and remove to a bowl. Add the remaining oil and fry the onions, carrot and celery until softened and then add in the mushrooms, tomato puree, salt and herbs and fry for 1 more minute. Return the lamb shanks to the pot and add the Guinness and stock to completely cover the shanks. Bring to a simmer and then reduce the heat, cover the pot and simmer for 2-3 hours, stirring occasionally until the lamb is falling from the bone.
Strain the lamb and vegetables from the sauce, return the sauce to the stove and reduce by half. Strip the lamb meat from the bone and discard the bones. Whisk the butter and flour together with the reduced sauce over heat for 3-4 minutes and then return the lamb meat and vegetables back to the sauce (removing any herb twigs and the bay leaves). Set aside.
To make the champ, peel the potatoes quarter them and cover with cold, salted water. Bring to the boil and simmer until tender. Drain the potatoes very well and mash or pass through a sieve until smooth. Whisk together the butter and cream over heat until emulsified and whip the emulsion into the potato. Stir through the spring onions and season to taste.
Preheat an oven to 180 degrees Celcius. Line a small individual pie tin with puff pastry, prick the base weight down with dry beans or pastry weights. Bake for 15 minutes until the pastry is cooked and golden brown.
Fill the pastry case with the lamb shank mixture and top with a large quenelle of the champ. Serve with a dressed watercress salad.