Cook the potatoes in a saucepan of lightly salted boiling water for about 10 minutes until just tender, then drain.
Meanwhile, dot the butter over the trout, season and cook under a preheated hot grill for 7–10 minutes until cooked through. Remove from the grill and set aside, keeping the grill on. Cook the bacon under the grill until crispy. When the fish is cool enough to handle, discard the skin and break the flesh into large flakes.
Heat 2 tablespoons of the oil in a large frying pan. Add the onion and cook for 5 minutes until softened. Add the remaining oil to the frying pan and add the potatoes. Cook until browned and crisp all over. Break the bacon into pieces, add to the pan with the trout and heat through. Serve scattered with the parsley.