Heat up a medium cooking pan and add oil. Season mushrooms and onions with some salt and pepper. Saute them over medium heat until just done. Drain off the mushroom juices.
Combine sour cream, heavy whipping cream, garlic powder, salt, and pepper in a bowl and mix well. (Salt to taste but remember you salted mushrooms as well.)
Add creamy mixture to the mushrooms, stir well and heat through.
Transfer mushroom mixture into a baking dish and spread shredded cheese over the top. Bake for 10-15 minutes.