For the potatoes, put a large pan of water on to boil. Halve any large potatoes and add them all to the pan, put a lid on and simmer over a medium heat for 15-20 minutes. When cooked, drain and add the butter and parsley. Mix well.
For the peas, tip the pancetta into a saucepan. Cook over a high heat until they start to crisp, then add the chopped onion and turn the heat down to low. After 2 minutes tip in the cream and peas and cook for another 3-4 minutes, until the peas are cooked. Season with salt and pepper.
For the fish, heat the oil in a frying pan. Season the fish with a little salt and pepper and dust with the flour. When the oil is hot, add the fish and cook for about 3 minutes on each side, until golden and flaky.
To serve, spoon the peas into bowls and top with the potatoes and fish.