Sir Stew is considered the hardest to find of the trio that you need to investigate for this side quest. From Sir Mash, that on up the stairs leading to the Nefarious Battlements. You can head on up to the Battlement or just drop off the far side to the miner you see below you.
Once there, clear out the Soldiers that present a threat then look down over the edge. You will see a lip in the cliff below with someone mining on the side of the mountain. Drop down to them to find Sir Stew.
Head over to the chest by him and investigate it.