In a large Dutch oven, heat the oil over medium heat until it begins to glisten. Stir in the onions, garlic, carrots, celery and fresh rosemary, and cook until the onions are translucent, about 5 minutes. Add the mutton and stir until all sides of the meat have browned. Add the wine and tomato paste and allow to cook for about 10 minutes. Then add the vegetable stock or water, reduce heat to medium-low and allow to simmer for 4 to 6 hours, periodically checking to make sure the liquid doesn't dissipate, and adding water if needed. When the meat is beginning to become tender, add the potatoes. Season with salt and pepper. The stew is finished when the meat is very tender (the flavor should be intense and very tasty). Adjust the seasoning, as needed, and whisk in the butter just before serving to make the sauce a little richer.