To Make the Mash: Place the potatoes in a pot of water and add the salt. Bring to a boil, lower the heat to a steady simmer and cook for about 15-20 minutes or until they potatoes are just tender when pierced with a knife. Be careful not to over-boil them. Thoroughly drain the potatoes and place them back in the empty pot set over very low heat just to maintain warmth. If using a ricer or food mill (this creates the most optimal texture), run the potatoes through it now while they're hot. If using a hand masher mash the potatoes while they're hot. (Avoid using an electric mixer or food processor. See blog post for explanation.) Mash the potatoes until fluffy and you've reached the desired degree of smoothness. Be careful not to over-mash or the potatoes will become gluey. Use a spoon to stir in the butter. Once melted stir in the hot milk gradually, allowing time for the potatoes to absorb the liquid after each addition. Add more hot milk as needed to achieve desired consistency. Season with salt and pepper to taste.
To Prepare the Sausages: While the potatoes are boiling place the sausages in a baking dish with a little oil and roast the sausages for about 10 minutes on each side or until nicely browned. (Alternatively you can fry the sausages but baking them creates an especially nice crispy outer shell with a juicy interior.) For sausage-making enthusiasts we highly recommend our Homemade British Bangers.
To serve, place a mound of mashed potatoes on each plate, lay the sausages on the mashed potatoes and top with onion gravy. Peas are a traditional side.