01 - Heat the oil in a large skillet over medium heat. Add the sausages and cook, turning occasionally, for 10-12 minutes until browned on all sides and cooked through. Remove sausages from the pan and set aside, keeping warm.
02 - In the same pan, melt the butter. Add the sliced onions and cook over medium-low heat for 12-15 minutes, stirring often, until soft and golden brown.
03 - Add the garlic and cook for 1 minute until fragrant.
04 - Sprinkle the flour over the onions and stir well, cooking for 1-2 minutes to make a roux.
05 - Gradually pour in the stock, stirring constantly to prevent lumps. Add Worcestershire sauce, mustard, thyme (if using), and season with salt and pepper.
06 - Bring the gravy to a simmer and cook for 3-5 minutes until thickened.
07 - Return the sausages to the pan, nestling them into the gravy. Simmer gently for 5 minutes to heat through and blend flavors.
08 - Serve the sausages and onion gravy hot over mashed potatoes. Garnish with chopped parsley if desired.