Place a large dutch oven over medium heat. Add about half of the butter, and when it is melted, begin browning the meat without crowding the pan. (This will require at least two batches.) Be sure to season the meat with salt and pepper as you add it to the pot. Set the browned meat aside.
Add the remaining half of butter along with the chopped onion and minced fennel (if using) to the pot. Saute until the onion begins to look translucent, about 5 minutes.
Add the tomato paste and sugar. Stir and cook for about a minute more.
Return the meat to the pot and sprinkle over the flour. Stir and cook for about 2 or 3 minutes more.
Add the Guinness and stir to loosen the brown bits in the bottom of the pot before adding the beef broth (or water and bouillon.) Bring the mixture to a boil and then reduce the heat to a simmer.
Using a piece of butcher's twine, tie the rosemary, bay leaf, and parsley into a little bundle to make a bouquet garni.
Add the potatoes, carrots, and the bouquet garni. (Tie the loose end of the butcher's twine loosely around one of the pot handles before pressing the bouquet garni down into the stew.)
Simmer the stew for about 1½ to 2 hours (uncovered) or until the lamb is tender and a fork slides easily into the potatoes and carrots. Then remove the bouquet garni. Taste the stew for seasoning and add salt and pepper as necessary.
Serve the stew garnished with a sprinkle of chopped flat leaf parsley.