Butterfly the pork loin, basically cut the pork down the center, without completely cutting through, so when you open the two halves they resemble a butterfly. Use your meat mallet to flatten it a bit.
Heat 1 tbsp of the olive oil in skillet. Add chopped onion and saute until onion is translucent. Add chopped mushrooms and continue cooking for another 3 to 5 minutes. Season with salt and pepper and stir in oregano and rosemary. Set aside.