Meanwhile, 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, but stirring occasionally for 12-15 minutes until soft and beginning to take on a little colour.
Combine
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.
Serve
Divide the pasta between bowls and sprinkle over the lemon zest and more chilli flakes.
Serve.
Suggestion: Stir through a tin of chickpeas for a bit of protein and texture.