Melt the butter in a large pot, then when it’s hot fry the bacon until cooked and crisp. Remove the bacon from the pot and put aside.
Fry the onion and garlic for a few minutes until softened and fragrant, then add the potato and stock and let simmer for about 10 minutes until the potato is cooked.
Add 1 can of the cannellini beans and the spinach, then stir and cook for a couple of minutes until the spinach has wilted into the liquid. Add about half of the cooked bacon and any juices that have drained from it, the mint, lemon, nutmeg and pepper, then remove from the heat and blend with a handheld blender or in batches in a food processor until smooth (I used a handheld blender like this one and it was super easy!)
When smooth, add the second can of beans and the rest of the bacon (reserving a tiny bit of each for decoration), then serve out into bowls. Top with a little Greek yoghurt and chopped chives, if you like.