You can make the Caramelized Onions & Mushrooms a day or two ahead and store them in the fridge or make them while you cook the pork tenderloin.
For the mushrooms, you can either buy them already sliced or whole and slice them yourself.
Peel the onion, slice in half, then thinly slice. Slices should be about the width of the chefs knife.
Once they are all sliced, separate the onions with your fingers.
To a large preheated skillet, drizzle the olive oil and then add the onions. Stir to coat in the oil, add salt and pepper. Cook the onions on medium-low heat for 20 minutes, stirring every 5 minutes or so.
Push the onions over to one side of the skillet and add the Ghee or butter to the open space and then the mushrooms. Saute the mushrooms for 10 minutes until browned.
Then mix the onions and mushrooms together, add the oregano and balsamic vinegar, and cook down for a final 10 minutes.
At this point, you can keep it warm or save it in a glass storage container in the fridge so that you can reheat it later.
Make the pork spice rub by adding to a small mixing bowl, the Dried Oregano, Garlic Powder, Onion Powder, Salt, and Pepper. Stir to combine.
Spray or rub all sides of the pork tenderloin with extra virgin olive oil, then sprinkle and rub the seasoning all over the outside. Then place the tenderloin under the broiler for 5-8 minutes until the top is nicely browned. Move the pork down to the center rack and cook at 400 F degrees for 10-15 minutes or until the internal temperature reaches 145 F.
When the pork tenderloin is done cooking, let it rest for 10 minutes tented with aluminum foil to allow the juices to redistribute. Then slice the pork and serve with the Caramelized Onions and Mushrooms.