Heat the oil in a frying pan over medium heat. Add the garlic and onion and saute until fragrant and onion is just softened. Add the harissa paste and stir through for another minute.
Add the finely chopped tomatoes and oregano, and season with salt and pepper. Allow to simmer, stirring frequently, for a good 5 minutes. You should feel the sauce starting to thicken slightly.
Add the butter beans to the pan, along with the liquid from ONE of the cans of beans and stir well. Bring everything back to a simmer, and allow to cook for 10-15 minutes. The sauce should reduce a little more and thicken.
While the sauce is simmering, combine half of the fresh herbs in a small bowl along with the greek yoghurt.
Once the beans are ready, stir through the remaining fresh herbs. Top with a generous dollop of yoghurt, a squeeze of lemon, and plenty of crusty bread.