Pour the stock into a large saucepan and add the onion and chicken breasts. Season, bring to a gentle simmer (only a few bubbles) over a low heat and cook for 10 mins or until the chicken is cooked through. Remove the chicken and set aside to rest for 5 mins, then finely shred.

Meanwhile, add the butter beans with their water to the pan (this will help the soup thicken slightly) and stir through the 1 tbsp pesto. Simmer gently for 5 mins, then add the shredded chicken and spinach. Cook for a further 2 mins until the spinach has wilted.

Divide between 4 bowls and top each bowl with 1 tsp of pesto and a little grated Parmesan, if you like.