In a small saucepan, combine the potatoes and enough salted cold water to cover. Bring to a boil, then lower the heat and simmer until tender, about 15 minutes. Drain, return to the pot and mash with 6 tablespoons cream, 1 teaspoon salt and a pinch of pepper. Set aside.
Meanwhile, in a medium bowl, combine the ground sirloin with 1 1/2 teaspoons salt and 1/2 teaspoon pepper. Shape into 4 oval patties about 1/2 inch thick. Coat with the flour.
In a large, heavy skillet, heat the olive oil over medium heat. Add the ground sirloin patties and cook until browned, about 2 minutes on each side for medium-rare. Transfer to a plate and cover with foil to keep warm.
Add the onions to the skillet and cook over medium heat, stirring, until slightly softened, about 5 minutes. Pour in the beef broth, steak sauce and remaining 6 tablespoons cream and bring to a boil. Cook, stirring occasionally, until the sauce thickens, about 4 minutes.
Meanwhile, cook the peas according to package directions.
Return the meat patties to the skillet and let simmer in the sauce for about 4 minutes. Arrange the patties on plates and spoon the sauce on top. Serve the mashed potatoes and peas alongside.