BANGERS: Place the sausages in a roasting pan or rimmed baking sheet, drizzle with the olive oil and toss to coat. Arrange sausages in a single layer and bake, turning them after 15 minutes, cook until evenly colored, about 30 minutes.
MASH: Meanwhile, peel the potatoes and cut them into chunks. Place potatoes in a pot of salted water; bring to a boil over medium-high heat. Cook until the potatoes are tender about 10-15 minutes. Drain, cover with a kitchen towel and let stand for about 5 minutes.
In the same pot over medium-high heat, warm the milk and butter. Remove from the heat and set aside.
Pass the potatoes through a ricer OR place in a bowl and mash. Add the warm milk mixture, stir or beat until smooth; season with salt and pepper to taste.
ONION GRAVY: Meanwhile, in a nonreactive skillet over medium heat, melt the butter with the olive oil. Add the onions and cook, stirring frequently, until they become translucent, about 10 minutes.
Stir in the flour and cook until lightly colored, about 2 to 3 minutes. Stir in the vinegar, red wine and stock, increase the heat to medium-high and bring the mixture to a boil. Then reduce the heat to low and simmer until a rich sauce forms, about 10 minutes; season with salt and pepper to taste.
TO SERVE: Mound about a 1/2 cup of mashed potatoes onto 4 individual plates. Top with sausages and ladle on onion gravy. Pass remaining gravy at the table.