Cover potatoes, rutabaga and celery root with water in a medium sauce pan and cook over medium high heat about 20 minutes until they are fork tender.
Brown the ground meat in a large skillet over medium high heat. Add the salt, pepper and curry after the meat is brown and cook another 3 minutes. Drain off any excess fat.
When potato mix is cooked, drain and mash them thoroughly. Add the baking powder, butter and warm mix stirring well. I actually beat mine with electric beaters until they are really nice and smooth textured.
In a deep casserole dish layer the ground meat, peas and top with the mashed potatoes.
Bake 25-30 minutes at 350 degrees.
If freezing allow the casserole to thaw and bake approx 60 minutes at 350 until heated through.
You can also hold assembled casserole in the fridge and reheat the following day. It will take 45-60 minutes to heat through if cold from the fridge.