Heat butter in a small saucepan over a medium high heat until melted and starting to bubble.
Crumble the stock cube into the butter and stir. Mix in the flour and cook for 1 minute, stirring constantly.
Remove pan from the heat and whisk in the water and milk. Return pan to the heat and stir until thickened - about two minutes.
Remove from the heat, stir in the cheese until melted and pop the lid on to keep warm.
Heat the sunflower oil in a saute pan.
Gently fry the onion until it softens, about five minutes. Add the garlic and the grated courgette, season well and saute for about ten minutes, until the moisture in the courgette has evaporated and the vegetables have softened.
Meanwhile, cook the spaghetti in boiling, salted water according to the packet instructions - about ten minutes.
When the spaghetti is cooked, drain and stir the sauce and vegetables through. Serve immediately with parsley to garnish.
For a meaty version use a chicken stock cube, Parmesan or Grana Padano cheese and stir in a few slices of chopped salami, pepperoni, ham or bacon. This is the version my husband prefers!
Mum of three, daydream adventurer, OU Life Sciences graduate, occasional crafter, ex-pat Canadian & quite possibly Britain's most northerly food blogger.