Put the potatoes in a medium saucepan and add enough water to cover; season with salt. Bring to a boil, then reduce the heat to a simmer and cook until the potatoes are tender, 12 to 15 minutes. Reserve 1/2 cup cooking water, then drain the potatoes and return to the pan.
Meanwhile, season the steak generously with pepper and the seasoned salt. Heat the vegetable oil in a large skillet over medium-high heat. Add the steak and cook until well browned, about 4 minutes per side for medium rare (reduce the heat if necessary). Transfer the steak to a cutting board.
Combine the milk, cheese and butter in a small saucepan over medium heat. Cook, stirring, until melted and smooth. Pour into the potatoes and mash, adding some of the reserved cooking water as needed to loosen. Stir in the white and light green parts of the scallions and season with salt and pepper. Cover to keep warm.
Heat the peas as the label directs. Slice the steak and divide among plates along with the mashed potatoes. Top the potatoes with the peas and scallion greens. Serve with steak sauce.