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