Heat a drizzle of oil in a saucepan over medium low heat. Gently fry your onions until lightly browned, then add the sugar and butter and caramelise the onion for 10 to 15 minutes, or until browned and sticky.
Sprinkle the flour over the onions and stir for another 2 minutes (this thickens the gravy).
Now pour in your previously boiled water and crumble over the stock cube. Gently stir and then add the dash of Worcestershire sauce.
Simmer over a low heat and stir occasionally for 10 to 15 minutes, or until thick. Season to taste before serving.