Bring 1.5 litres of stock to the boil.
Add potatoes and simmer for 4-5 minutes.
Add carrots, leeks, and oats then boil for another 4-5 minutes until tender.
Crush the vegetables in the pan using a potato masher.
Reduce heat to a simmer and stir in 400g of grated cheese until melted.
Season with salt and pepper and simmer for 3-4 minutes.
Butter one side of each bread slice and create sandwiches with the sliced cheese.
Heat remaining butter and oil in a frying pan over medium heat.
Fry sandwiches for 2-3 minutes per side until golden and the cheese melts.
Ladle soup into bowls and garnish with extra leeks and watercress.
Slice the toasted sandwiches into squares and serve with the soup.