Preheat oven to 400°F.
Place a large skillet or pan over medium low heat.
Add oil and heat until shimmering.
Add ground beef and start to cook, breaking up meat with a wooden spoon.
Add 1/2 way through the cooking, add mushrooms and onions. Continue to cook until meat is browned and mushrooms and onions are tender.
Reduce heat to low, Add cream of mushroom soup and sour cream. Simmer 5 minutes until heated through.
Place mixture into a 13 x 9-inch casserole dish.
Top with corn. Spread mashed potatoes over corn. (Yeah, it's a little tricky. Just do you best - it will still taste good.)
Sprinkle with cheddar cheese.
Cover with foil.
Place in preheated oven and bake for 30 minutes.
Remove from oven and, using a spatula, place helpings into individual serving dishes, trying to maintain the structure of layers (So it looks good! ;) )
Serve hot.