First make the mashed potatoes. Peel the potatoes and cut them into 1/2-inch cubes.
Put them in a saucepan and cover with water, then bring to a boil.
Boil until they are done, about 10-15 minutes.
Drain the potatoes and add the milk, butter and salt. Mash until they reach the desired consistency, and then set aside while you make the filling.
Preheat the oven to 400 F.
Heat a frying pan over medium heat and add the olive oil.
Cut the lamb into small pieces and sprinkle with salt and pepper.
Brown in the olive oil and remove from pan when browned on all sides. Set lamb aside for a few minutes.
Add the butter to the frying pan along with the onion, rutagaba, carrots, celery, garlic, thyme, rosemary and water. Bring to a boil and simmer for about 15 minutes.
Add the lamb, tomato paste, flour, and stock to the pan and bring to a boil, and simmer for 10 minutes to let the sauce thicken slightly.
Spoon the filling into a large casserole dish or multiple small ones.
Top with mashed potatoes, spreading them all the way to the edges of the dish so the filling won't bubble out. It is probably a good idea to put the casserole dishes on a tray, just in case some filling spills over as it cooks.
Bake in a 400 degree oven for 30 minutes, then broil for 2 minutes to brown the potatoes.