Remove the silver skin from the pork tenderloin with a sharp knife. Slice into 1.5 - 2 inch disks. Flatten slightly with the palm of your hand. Season with salt and pepper.
Prepare the mushrooms, onion, garlic, thyme and parsley.
Heat oil in a large skillet to medium-high. Brown pork medallions in 2 batches, (or more depending on the size of your pan) cooking approximately 2-3 minutes per side, or until nicely seared. Remove to a plate and set aside.
Lower heat to medium-low. Add a splash more of oil if needed. Add sliced onions. Cook, stirring, for 3-4 minutes or until starting to soften.
Add mushrooms, garlic, and thyme. Stir and cook for 5-7 minutes or until mushrooms are soft. Stirring occasionally.
Add white wine and Dijon mustard. Turn heat up slightly and let simmer for 2-3 minutes to reduce slightly.
Add cream and stir. Add pork back to pan with any accumulated juices.
Bring to a boil and let simmer for approximately 5 minutes, until pork reads 145 degrees F on a meat thermometer or is just slightly pink in the center.
Sprinkle with fresh parsley and serve.
Store leftovers in the fridge for 3 days.