In a large, heavy bottomed pot, heat oil. Add the lamb and saute until browned on all sides, about 5 minutes. Remove lamb to a plate. Add the onion and saute until soft. Add the garlic and saute for 2 more minutes. Season with salt and pepper. Add the butter and melt. Add the flour to make a roux. Lower the heat and cook the roux until it is brown, about 15 minutes. Whisk in the wine and stock and raise the heat to medium high. Add in the reserved lamb. Add the mustard. Bring to a boil and reduce the heat to low. Cook over low heat for about 30 minutes, or until liquid is slightly reduced and thick. In a separate saute pan, saute the mushrooms until wilted. Add the mushrooms to the stew. Add the potatoes and simmer another 30 minutes, or until potatoes are tender. Add the thyme. Season with salt and pepper. Garnish with toasted pumpkin seeds.