Preheat oven to 350 degrees.
In a large bowl, combine cooked sweet potatoes, butter, and ½ cup milk. Beat until combined. Add additional milk if needed.
In a large skillet, brown the ground beef with the mushrooms and onions, crumbling as it cooks. Drain well.
Add remaining ingredients to the skillet with the beef, except for the mashed sweet potatoes. Bring to a simmer.
If you are using a cast iron skillet spoon the mashed sweet potatoes over beef mixture. If not you will need to transfer the beef mixture to a 2-quart baking dish before add sweet potato topping.
Next you will smooth the sweet potato topping. Bake for 25 to 30 minutes or until bubbly.
Sprinkle with smoked paprika or chopped parsley as garnish if desired.