01 - Heat vegetable oil in a large skillet over medium heat. Add sausages and cook for 10-12 minutes, turning occasionally until browned on all sides and cooked through. Remove from pan and set aside, keeping warm.
02 - Melt butter in the same pan. Add sliced onions and cook over medium-low heat 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 remove raw flour taste.
05 - Gradually pour in stock while stirring constantly to prevent lumps. Add Worcestershire sauce, Dijon mustard, dried thyme, salt, and black pepper. Bring to a simmer.
06 - Simmer gravy for 3-5 minutes until thickened to desired consistency.
07 - Return sausages to the pan, nestling them into the gravy. Simmer gently for 5 minutes to heat through and blend flavors.
08 - Serve sausages and onion gravy hot over mashed potatoes. Garnish with chopped fresh parsley if desired.