Cook your pasta in a pot of well salted boiling water, reserving a cup of the pasta water.
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.
Remove from the pan and then fry the onion gently for 3 - 4 minutes until translucent and softened.
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 and 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.
Feel free to finish the sauce with a tablespoon of butter.
Serve with freshly grated parmesan, extra rosemary and lemon zest.
