Melt the butter in a heavy bottomed pot and saute the onions until soft and translucent. Season well with salt and freshly ground black pepper.
Add half of the cubed potatoes and 2 cups of stock, bring to the boil, then reduce the heat and simmer for about 10 minutes. When the potatoes are soft, blend the soup with a stick blender until smooth.
Add the rest of the potatoes and stock and cook for another 5-10 minutes or so until the potato pieces are soft.
Stir in the shredded tuna and the green peas (or sweetcorn) and simmer for another minute to make sure that everything is heated through. Taste and add some more salt and pepper if necessary.
Serve hot with some crusty rolls for lunch or with some pizza for dinner.
Notes
You can use chicken stock in this recipe if you don't have fish stock.
This soup will keep in the fridge for 2-3 days and can also be frozen.
If you use canned peas, corn and stock this soup can be made entirely from pantry staples.