Add the butter to a saucepan over medium heat. When the butter is melted and bubbling add the onions and cook, stirring often until softened, not browned.

Add the flour into the butter to the onions and mix well to combine. Cook, stirring for a minute or so to cook out the raw flour taste.

Add the broth, Guinness and Worcestershire sauce and stir. Whisk in the tomato ketchup until well mixed. Bring to a low simmer until thickened. Add salt and ground black pepper to taste. If it tastes a little bitter from the Guinness, add a tiny pinch of sugar.