Preheat your oven to 200°C (390°F).
In a pan, heat ghee over medium heat. Add onions, garlic, and mushrooms. Cook until the mixture is dry. Season with salt and pepper.
Sear the beef tenderloin in a hot pan until browned on all sides. Let it cool.
Lay out the prosciutto slices on a piece of plastic wrap, overlapping slightly. Spread the mushroom mixture over the prosciutto.
Place the beef on top and roll it up tightly using the plastic wrap. Chill in the fridge for 15 minutes.
Roll out the paleo puff pastry and wrap it around the beef. Brush with beaten egg.
Bake for 25-30 minutes, or until the pastry is golden brown and the beef is cooked to your liking.
Let it rest for 10 minutes before slicing.