Heat the butter and olive oil in a large frying pan over a medium heat
Add the leek, garlic, chilli flakes and a pinch of salt, then cook with the lid on, stirring occasionally for 12-15 minutes until soft and beginning to take on a little colour
Stir in the miso paste and pasta and toast for a minute or two
Add in the vegetable stock and bring to a simmer
Cook with the lid off until the pasta is al dente, adding a little more water if it reduces too quickly
Stir in the lemon juice
Taste and season with salt, pepper or more lemon
Divide the pasta between bowls and sprinkle over the lemon zest and more chilli flakes
Serve
