Add 3/4 cup chopped onion, rosemary, 6 cloves garlic, brown sugar, coarse salt, lemon zest, lemon juice, ground mustard, and the 1 tablespoon black pepper to a food processor and chop for 30 seconds or until well blended.  Add the brisket to a large casserole and rub the marinade all over.  Cover and refrigerate overnight.

Preheat the oven to 350º F.

Add the remaining 2 cups chopped onion to the casserole with the brisket, along with the turnip and carrot.  Add enough beef broth until 3/4 of the brisket is submerged.  Cover with aluminum foil and set on a rimmed baking sheet and place in the oven.  Braise the brisket for 3 hours, or until tender.

Using a small spoon, combine the softened butter in a small bowl with the flour, minced garlic, parsley and salt and pepper until uniform and set aside. 

Once the brisket is done, transfer the brisket and vegetables with a slotted spoon to a platter and cover to keep warm. Transfer the braising liquid to a large sauté pan and bring to a boil.  Whisk in the butter mixture to thicken the sauce and simmer for 1 minute.  Remove from the heat. Taste and adjust the seasoning if necessary.

Serve the brisket with the buttered noodles, vegetables and sauce.