In a large 12 inch skillet heat 2-3 Tbsp oil over medium-high heat. To the pan add sausages and cook turning until browned on all sides and cooked through. Around 6-8 minutes. Remove from the pan and keep warm.
Add butter to the skillet, lowering the heat to medium. Add the onion to the pan. Saute until softened and beginning to brown scraping any brown bits from the bottom of the skillet for 6-8 minutes. Add the garlic, cook for 1 minute longer.
Add the flour to the onions stirring until absorbed. Cook bubbling gently while stirring constantly for 1-2 minutes.
Whisk in the beef stock, Kitchen Bouquet, Worcestershire sauce, salt and black pepper. Cook over medium heat until thickened, adding more stock to thin, if needed. (This isn't a thick gravy, it will thicken more after it cools.) Add the fresh thyme, mix well.
Serve sausages over mashed potatoes drizzled with onion gravy garnished with parsley and a side of green peas.