Put the sweet potato cubes in a pot of cold salted water, bring to a boil over high heat and simmer over medium heat until very tender, about 15 minutes. Drain and mash with the milk, salt and pepper.
Preheat the oven to 220°C (425°F) with a rack in the upper third. Line a baking sheet with parchment.
Blitz the bread into crumbs, then mix with the parmesan, chopped parsley, lemon juice and oil.
Pat the haddock dry, lay it on the tray and press the crumb mixture firmly over the tops.
Bake for about 12 minutes, until the crust is golden and the fish is opaque and flakes easily, 63°C (145°F) in the thickest part.
Meanwhile, cook the peas in a little boiling water over high heat for 3 minutes and drain.
Serve the haddock on the sweet potato mash with the peas.
Tips
Swaps
Gluten-free: crushed gluten-free crackers for the crust.