Heat the butter in a saucepan, add the potatoes and leek and cook for 5 minutes to soften. Add the stock and bring to the boil, then reduce the heat, cover and simmer for 20 minutes until the potatoes are tender, adding the peas for the final 5 minutes.
Blend the soup with a stick blender or in a food processor until smooth. Heat through and season.
Toast the bread slices on one side under a preheated grill, then turn the slices over, top with the cheese and grill until melted.
Ladle the soup into 2 warm serving bowls, swirl through the pesto and serve with the toasts.