Stew gravy

Peel the onion and grate it on a fine grater. Heat the butter in a frying pan until the foam pulls away and let the butter turn brown while stirring.

Stir in the grated onion, flour and milk. Heat everything with moderate heat until the mixture starts to caramelize and becomes evenly browned.

Stir in the mustard and balsamic vinegar and add the stock. Bring the gravy to the boil while stirring and let everything boil well to a nice gravy.