Heat the oil in a medium saucepan. Add the onion and potato. Season well and cook for 5 minutes over a medium heat. Add the peas, then the stock and bring to the boil, then add the watercress and mint. Simmer for 10 minutes.
Remove from the heat and blend the soup with a hand blender or in food processer until smooth (you may need to do this in batches). Return the soup to a clean pan, cover with a lid and keep warm until ready to serve
In a small bowl, mix the horseradish with the crème fraîche and season. Serve the soup in bowls with a swirl of the horseradish cream, a good grind of black pepper and some pieces of crusty bread, if you like.
Freezing and defrosting guidelines
Make the soup, then leave to cool at room temperature. Freeze (without garnishes or toppings) in a rigid container, leaving a bit of space for expansion, for up to 1-3 months. Reheat either from frozen or defrost in the fridge overnight. Once piping hot, add toppings or garnishes and serve.