Preheat oven to 180 degrees celsius.
Cut the fillet into medallions the width of a slice of prosciutto.
Spread 1 teaspoon of Dijon mustard on each of the prosciutto slices. Then wrap the prosciutto around the fillet, with the mustard on the inside. Tie the medallions up with kitchen string.
In a pan on high heat, sear the two flat surfaces of the fillet medallions until deep brown in colour. Then place the fillets in a lined tray in the oven, or place the oven-safe pan in the oven for 10 minutes.
In the meantime, slice the mushrooms into roughly 1 cm thick slices and saute on high heat with the garlic, a splash of oil and a tablespoon of butter. Season with salt and pepper. Add the port off heat.
Remove the fillets from the oven and allow to rest for 5-10 minutes
Mix the softened butter with the chopped parsley until well combined.
Slice the pork fillet and serve with the mushrooms and butter.