Place 10g butter in a sauteuse.
Add the onion and cook for 4-5 min without color.
Add the beef and cook,coloring lightly.
Add the demi-glace.
Simmer until tender and correct the seasoning.
Meanwhile cook the spaghetti in plenty of salted boiling water.
Allow to boil gently.
Stir occasionally with a wooden spoon.
Cook for approx. 12-15 min.
Drain well in a colander.
Return to a clean pan containing 10g of butter.
Correct the seasoning.
Serve in a dish with the sauce in the centre of the spaghetti.
Serve grated cheese separately.