Place the potatoes in a pan with just enough water to cover them. Bring to the boil, then cover and simmer for 20 minutes or until tender.
While the potatoes are boiling, heat the olive oil and 1 tsp of the butter in a frying pan over a medium heat, until sizzling. Season the coley with salt and black pepper and add to the pan. Cook for 3-4 minutes on each side – when cooked, it will be white all the way through – then keep warm and set aside.
When the potatoes are cooked, bring the milk to the boil. Drain the potatoes then return to the pan with the milk and remaining butter. Mash until smooth and creamy. Season with salt and pepper to taste.
Divide the mash between two plates then top the with fish, garnished with the parsley, and serve with the green veg and lemon wedges.