Bring a large pot of water to a rolling boil. Add 1 teaspoon salt and the potatoes. Cook until the potatoes are fork tender.
While the potatoes are cooking, combine the milk and butter in a microwave safe measuring cup with a spout. Microwave on medium power until the butter starts to melt and the milk is warm. Set aside and keep warm.
Drain the potatoes well in a colander then return to the pot. Using a potato masher, smash the potatoes until smooth. Add the warm milk and butter mixture and blend until combined. Add salt and pepper to taste. Cover and keep warm until needed.
To prepare the bangers and onion gravy:
Heat a large skillet over medium heat. Add 1 teaspoon vegetable oil and the sausages. Cook the sausages are cooked through and dark brown on all sides. Remove to a plate and keep warm.
Add the remaining 1 teaspoon vegetable oil to the now empty skillet. Add the onions and 1/4 teaspoon salt. Cook, stirring occasionally until the onions are browned, about 6-8 minutes. Stir in the garlic, tomato paste and thyme and cook until fragrant, about 1 minute. Add the flour and cook for 1 minutes, stirring constantly. Add the beef broth, a little at a time then the Worcestershire sauce. Scrape up any browned bits from the bottom.
Bring the gravy to a simmer then reduce the heat to medium-low. Simmer, stirring occasionally until the gravy has thickened, about 3 to 5 minutes. Season with salt and pepper to taste.
At this point you can add the sausages to the gravy and serve over the mashed potatoes, or divide the mashed potatoes and sausages onto individual plates and spoon the gravy over the top. Garnish with chopped parsley and serve.
We like russet potatoes for this recipe. Do not use an electric mixer to mash the potatoes. It will cause them to be gummy, not fluffy.