In a large stockpot, place the potatoes and a generous pinch of salt. Cover with cold water and bring to a simmer. Cook at a simmer until the potatoes are fork-tender, about 15 minutes. Drain the potatoes and return to the pot. Add the butter, milk, buttermilk, salt and pepper. Mash until smooth. Fold in the scallions.
In a large saucepan, heat the oil for the gravy. When hot, add the mushrooms, onion and garlic and cook until caramelized. Season with thyme, salt and pepper. Remove the vegetables to a bowl.
Return the saucepan to medium heat with the butter. When melted, add the flour, a little at a time, whisking constantly. Cook for 2-3 minutes, until the roux starts to brown. Whisk in the beef stock until smooth, then bring to a simmer until there is no more raw flour smell or taste. Fold in the mushrooms and onions. Season to taste. For a thicker consistency, continue to simmer and reduce the gravy.
Place a steaming basket in a small saucepan with 1 inch of water. Add the carrots and peas and bring to a boil, covered. Steam for 5 minutes, or until soft.
Combine the celery salt and pepper in a small bowl. Shape the beef into patties and season both sides with the seasoning mixture.
Heat a large skillet or grill to high heat with a drizzle of oil. When hot, sear the patties until golden brown, 3-4 minutes. Flip and cook for 3-5 more minutes on the other side, until cooked to your desired doneness.
Divide the mashed potatoes between plates. Top with salisbury steaks, steamed vegetables and onion gravy. Garnish with parsley.