Heat butter in a large saucepan and add leeks. Cook, stirring often, over a llow heat 12-15 minutes. Add potatoes, stock and herbs, and bring to a boil. Simmer 15-20 minutes until potatoes are tender. Add peas and cook a further 5 minutes. Remove from heat, add Stilton, and season with salt and pepper.
Transfer mixture to a food processor and blend until reasonably smooth.
Can be kept in the fridge for up to a week.
For parsnip crisps, cut parsnips with a vegetable peeler into long thin strips. Deep fry in hot oil for 20-30 seconds until crisp and golden. Drain on paper towels. Once cooled crisps can be stored in an air-tight container for 1 week.