Place frying pan on high heat. Slice pork into 6 medallions, bashing to flatten slightly, and roughly chop the mushrooms.
Place them in the pan with 1 tbsp of olive oil and cook for 2 minutes each side or until mushrooms are golden and pork is just cooked through, then remove just the mushrooms to a plate.
Go in with the mustard and creme fraiche and cook for 1 more minute, loosening with a splash of water, if needed. Taste and season to perfection with sea salt and black pepper, then scatter over the golden mushrooms and arugula, and finish with a drizzle of extra virgin olive oil.