Boil the potatoes in a pan of salted water for 10-12 mins. Drain and leave to steam-dry, then mash until smooth. Season to taste.
Meanwhile, cook the haggis following pack instructions. Tip into a bowl and break into chunks with a fork, then fold through the mash with three quarters of the spring onion and leave to cool. Form into four potato cakes using your hands.
Put the breadcrumbs in a food processor and blitz until fine. Tip onto a plate, then coat the cakes in the crumbs. Chill for 15 mins.
Heat the oil in a large non-stick frying pan over a medium heat. Fry the potato cakes for 5-6 mins on both sides, or until golden.
Heat a large pan of water until just boiling. Crack one of the eggs into a small bowl. Swirl the water to create a whirlpool, then gently tip the egg into the centre of the pan. Cook for 2-3 mins until poached to your liking, then repeat with the remaining eggs.
Serve the cakes topped with the eggs and remaining spring onions.