Preheat the oven to 375ºF. Heat olive oil and butter in a large saute pan over medium heat.
Slice the onion into ribbons and place it in the pan, sprinkle with dried thyme. Allow the onions to cook over medium heat. Stir regularly and cook for 10-15 minutes or until they become golden brown.
Wash the mushrooms and remove the stems.
Spray a baking dish with non-stick cooking spray.
Place the mushrooms in the baking dish with a little separation between each one. Fill the mushrooms with tomato and cheese.
Top them with caramelized onions and place the baking dish into the oven.
Cook for 15 minutes or until the cheese melts.
Serve and enjoy.