Melt the butter in a large heavy-based saucepan. Add the onion, leek and celery and cook gently until the onion is soft, then season with sea salt and black pepper. Stir in the stock and cream.
Bring the soup to a simmer and cook for approximately 5 - 8 minutes until the vegetables are cooked. Add the peas and thyme, bring to the boil and cook for a further 5 minutes or until the peas are just tender. Be careful not to overcook them. Remove from the heat and blend the soup until smooth, season to taste with sea salt and black pepper.
To make the haggis croutons, toss the haggis in flour and then dip in the beaten egg and into the breadcrumbs until evenly coated. Place the croutons in the fridge until ready to use. Heat the oil to 180°C and deep fry the haggis croutons until golden and cooked – approximately 3 minutes – remove and drain well on kitchen paper. Serve the pea soup with the haggis croutons on top.