Drain the soaked split peas and put them into a saucepan with the parsnips, onion and stock. Bring to the boil and boil for 10 minutes. Reduce the heat, cover and simmer for 1 hour or until the split peas are soft.
Meanwhile, make the butter by dry-frying the cumin and coriander seeds and garlic in a small saucepan until lightly toasted. Mix into the butter with the coriander leaves and a little salt and pepper. Shape into a sausage shape on clingfilm or foil, wrap up and chill until needed.
Roughly mash the soup or purée in batches in a liquidizer or food processor, if preferred. Reheat and stir in half the coriander butter until melted. Add a little extra stock if needed then season to taste. Ladle into bowls and top each bowl with a slice of the coriander butter. Serve with toasted pitta breads.