Peel and finely chop the onion. Peel and finely dice the carrots. Heat a frying pan for 1 min over a medium heat. Add 1 tbsp oil and the chopped veg. Cook and stir for 5 mins till just softened.
Add the beef mince to the frying pan. Cook and stir for 4-5 mins till browned, using a wooden spoon to break up any lumps. Crumble the stock cube into a heatproof jug. Add 400 ml boiling water. Stir to dissolve.
Rinse the barley under cold water. Tip into the frying pan with the mince. Pour in the beef stock. Stir together. Cover and simmer for 30 mins.
While the mince is cooking, peel the potatoes and chop into chunks. Peel the swede and chop it into slightly smaller chunks. Place them in a large pan. Pour in enough boiling water to cover them. Pop on a lid and bring to the boil. Simmer for 15-20 mins till soft.
Meanwhile, peel and finely grate the horseradish. Swirl it through the pot of crème fraîche. Season generously with salt and pepper (if you have white pepper, use that).
Drain the potatoes and swede. Tip back into the pan. Mash together over a low heat to cook off any excess water. Season. Stir in most of the horseradish crème fraîche, keeping a few spoonfuls back for later. Take off the heat. Cover to keep warm.
Trim the tops and bottoms off the leeks. Rinse out any grit. Finely slice them. Heat a separate frying pan for 1 min, then add ½ tbsp oil and the leeks. Cook and stir for 4-5 mins till softened. Season.
Taste the mince and barley. Add more seasoning if you think it needs it. Serve with the horseradish mash, the leeks and a dollop of the remaining horseradish crème fraîche. Sprinkle over parsley leaves to garnish.