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 and set it aside.
Add the onion to the pot. Season with salt and 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.