Cook your pasta in a pot of well salted boiling water, reserving a cup of the pasta water. I try and time the cooking of the pasta so that I can transfer the pasta straight from the water into the sauce.
Heat the butter and olive oil over a medium heat in a large skillet that is big enough to fit all the pasta in at the end.
Fry the leek rounds for 4 - 5 minutes, turning once half way until both sides are golden brown. Be gentle when flipping them but it doesn't matter at all if they come apart.
Remove from the pan and then fry the onion gently for 3 - 4 minutes until translucent and softened. Add a drizzle of olive oil if it gets too dry.
Add the garlic and cook for a further minute.
Add the beans and their liquid into the pan and bring to a very gentle simmer.
Add the miso paste (you can mix it first with a couple of tablespoons of water to make it easier to mix in).
Add the lemon juice and rosemary, black pepper and mix well then let it gently simmer for a couple of minutes.
Taste for seasoning and add a good few cracks of black pepper and salt if needed.
Add your cooked pasta straight into the sauce with ¼ cup of pasta water and mix.
Add the leek rounds in and gently mix through and let it all very gently bubble away for a minute. Add a splash more pasta water if you need.
Feel free to finish the sauce with a tablespoon of butter.
Serve with freshly grated parmesan, extra rosemary and lemon zest.