1. Preheat oven to 175°C.
For the meat layer:
1. In a large frying pan over medium-high heat, saute onion in 25g butter. Add pot roast and cook until browned. Season with salt and pepper, to taste. Set aside.
For the potato layer:
1. Peel and slice potatoes 1 cm thick. Place in a saucepan, cover with cold water, and bring to a boil. Cook for approximately 15 minutes or until fork tender. Drain potatoes and transfer to a mixing bowl. Whip potatoes with an electric mixer, mixing until moderately smooth. Don't over beat them; a few lumps are nice.
2. Add 1/2 cup heated milk, 1 stick butter, and sour cream. Season with salt and pepper, to taste. Whip again until mixed. Adjust thickness by adding more milk, if desired.
For the biscuit layer:
1. Combine biscuit mix and 340 ml of milk. The mix should be thinner than that of normal biscuit mix but not runny.
2. Spray a 20 by 20 by 5 cm pan (or any similar casserole dish) with cooking spray. Layer mashed potatoes halfway up the sides of the dish. Next, spread a layer of corn or mixed vegetables on top of the potatoes. Then add the layer of pot roast and onions with gravy.
3. Pour the biscuit mix over the meat. Melt 60 of butter and drizzle over top. Bake for approximately 35 to 45 minutes or until top is golden brown. Let stand for a few minutes before serving.