Heat a little olive oil and knob of butter in a pan and gently cook the onions and garlic for 5 minutes until they start to soften. Add the potato and continue to cook for a further couple of minutes.
Add the stock and then bring to the boil. Simmer gently for 15 minutes until the potatoes are tender
Add the peas and simmer for another 2- 3 minutes.
Remove the pan from the heat and add the watercress and stir into the soup - this will wilt very quickly.
Blend with a stick blender until you have a smooth consistency. Season with salt and black pepper and keep warm.
Meanwhile, grill the bacon until crispy and drain on kitchen paper, break up into little pieces.
Ladle the soup into bowls and top with the crispy bacon, serve with crusty bread and plenty of butter.