Spray the pan with low calorie cooking spray.
Over a medium heat, sauté the onions for 5 minutes, until soft. Add the garlic and continue cooking for another minute.
Pour in the stock and oat milk, then add the potatoes and lemon juice. Stir well and bring to the boil.
Reduce the heat and allow to simmer gently for 20 minutes, until the potatoes are cooked.
Add the rocket, spinach and basil leaves and allow a minute or 2 for them to wilt.
Stir in the grated parmesan and then pour into a blender (or use a stick blender) and blitz to a soup.
Return to the pan, add a little salt and freshly ground black pepper to taste.
Return the pan to the heat to ensure the soup is piping hot and then serve in warm bowls.