Add the drained beans to a saucepan with the vegetable stock, and place over a medium-low heat. Simmer gently for around 5-10 minutes, until the beans are hot and very soft. Mash with a potato masher (I left mine a little chunky for texture, but you can make them completely smooth if you prefer).
Remove from the heat, and add the parmesan and pesto, along with plenty of black pepper (you probably won't need any salt unless you used low-sodium stock). Mix until well combined.
Serve immediately drizzled with a little extra virgin olive oil, if desired - or, keep in the fridge to reheat in the microwave later.