Preheat your oven to 350°
Cut your lamb roast into 8 to 10 big pieces and season liberally with salt and pepper; heat a few tablespoons of your favorite cooking oil in a large skillet set over medium-high heat. When the pan is nice and hot, add about half the pieces of meat and let them cook undisturbed until a nice golden crust forms, about 4-5 minutes. Flip the pieces of meat and cook until golden on that side, too.
Transfer the seared pieces of meat to a 7 quart Dutch oven and repeat with the remaining pieces of meat.
Once all the meat has been browned, return the skillet to the heat source and lower the heat to medium; add the sliced onions, garlic and mushrooms and cook for about 5 minutes, until slightly browned and softened. Add the rosemary, salt, cinnamon, coriander and white pepper and then pour in the bone broth. Stir well, making sure to scrape the bottom of the pan well so you get all the tasty bits to detach from it; pour over the reserved meat.
Add the water and dark beer (or use dark coffee if you want to keep this alcohol free), put the lid on and cook in the oven for about 2½ hours, or until the meat is tender and breaks up easily when pulled with a fork.
Stir in the pasta and return the dish to the oven for another 20 minutes, or until the pasta is just slightly undercooked.
Combine the sour cream, remaining dark beer (or more strong coffee) and cornstarch in a small container, whisk until evenly combined and add to the ragu. Stir well and return to the oven for a final 5 minutes, or until the pasta is cooked to your liking and the sauce has thickened nicely.
Garnish with a sprinkle of chopped rosemary and fresh parsley and serve without delay.