Heat the oil in a large saucepan over a medium heat. Add the leeks and gently fry for 7- 8 mins or until softened then add the garlic and fry for a further 2 minutes.
Pour in the hot stock and gently simmer for 6 mins. Add the watercress, reserving a few leaves for garnish, then the peas, and simmer for 5 mins.
Use a hand blender or processor and whizz until smooth. Stir through the lemon juice and zest, then season to taste. Stir through half the parsley.
Ladle into bowls and top with the remaining parsley, reserved watercress and a drizzle of olive oil.
Slice the Penny Loaf vertically into 2cm slices. Top with grated Carrigaline and pop under the grill for 2 minutes to melt. Gently place on the bowls of soup and sprinkle with a few watercress leaves.