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 fully cooked. Remove from pan and keep warm.
02 - Melt butter in the same pan. Add sliced onions and cook over medium-low heat for 12-15 minutes, stirring frequently, until softened 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 eliminate raw flour taste and form a roux base.
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 coating the back of a spoon.
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 fresh chopped parsley if desired.