Peel the onions and finely chop them. Peel the garlic cloves and crush or finely grate them. Slice the pepper in half, and use a spoon to scoop away the seeds and membrane. Slice the flesh into thin strips. Roughly chop the tomatoes.
Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the chopped onions, garlic, pepper and 1 tbsp smoked paprika. Sprinkle in a pinch of salt and pepper. Fry for 5-6 mins, stirring occasionally, till the veg are tender.
Stir the chopped tomatoes into the pan and continue to cook for a further 3-4 mins, till they start to soften.
Drain and rinse the butter beans and tip them into the pan. Crumble in the stock cube. Pour in 700ml boiling water and stir well to combine. Bring to the boil, then reduce to a simmer and cook for 10 mins.
Use a stick blender to whizz part of the soup, leaving it slightly chunky. No stick blender? Transfer a third of the soup to a blender and blitz, then stir back into the pan. Have a taste of the soup and add a little more salt and pepper if needed. Ladle the soup into deep bowls and serve.