In a large skillet over medium heat, drizzle in some oil. Sear the sausages, cooking on all sides until cooked through (about 10 minutes).
Alternately, you can sear them, and then place on a sheet pan. Bake at 375 degrees for about 8 minutes, or until cooked through while you make the gravy.
Turn the heat down to medium low. Add the butter to the skillet. Add in the onions and season with sugar and a pinch of salt. Cook until caramelized, about 20 minutes, stirring occasionally.
Sprinkle the flour on top, and stir. Pour in the beef broth. Add in the garlic powder, thyme, pepper, Worcestershire, mustard, and gravy master. Cook until thickened, about 5 minutes.
Serve the gravy over the sausages and mashed potatoes.