Heat oil in a large pan over medium heat, add sausages, and cook until browned all over. Remove and set aside.
In the same pan, add butter and onions, stir in sugar, and cook until the onions turn soft and golden.
Sprinkle flour over the onions, stirring to coat, then gradually add beef stock, stirring to avoid lumps.
Return sausages to the pan, add Worcestershire sauce, and let everything simmer for 20 minutes until the gravy thickens.
Season with salt and pepper, garnish with parsley, and serve hot with mashed potatoes or crusty bread.