Season lamb shoulder chunks with salt and pepper.
Heat oil in a large heavy skillet/pot over high heat. Cook lamb shoulder chunks until browned on all sides, working in batches if need be. Using a slotted spoon, remove the browned lamb from the pot & set it aside.
Add the onion to the pot. Season with salt & pepper. Saute over medium heat until slightly softened and edges are browning, about 5 minutes. Stir the butter into the onion until melted; add in the flour and stir until onions are coated. Cook about 1 minute.
Pour the stock/broth into the onion mixture and bring to a boil. Add the rosemary and garlic powder. Boil, stirring often, for 5 minutes.
Stir in the browned lamb, diced carrots and diced celery. Mix in the potatoes.
Bring the mixture to a simmer, reduce heat to low, cover the pot with a lid. Cook for 60-90 minutes.