Place all the duxelles ingredients into a food processor. Process until small pieces (do not puree). The goal is to achieve a finely chopped texture, not puree the mushrooms.
Lightly grease a skillet with butter. Add the duxelles and sauté until lightly browned and most of the water has evaporated. Set aside.
Lay a sheet of wide parchment or plastic wrap out on the counter. Place 4 or 5 slices of prosciutto, long sides overlapped by ½-inch or so, on the plastic wrap.
Spread the duxelles over the center of the prosciutto. Set aside.
Preheat a skillet over medium-high heat. Melt the butter then brown just the outside of the meat on all sides, including the top and bottom. Transfer to a plate. While the meat is still hot, brush it all over with the dijon mustard.
Place the meat on top of the duxelles, towards one end of the rectangle.
Roll the whole thing up using the parchment or plastic wrap to assist. Twist the ends of the plastic wrap tightly so that the Wellington looks shaped like a candy roll.
To make the carnivore pastry dough, place the mozzarella and the cream cheese in a heat safe bowl and microwave for 1-2 minutes or until the cheese is entirely melted. Stir well. Add the pork rind crumbs and egg. Use a spatula to combine well.
Roll the dough flat between two pieces of parchment paper.
Place the prosciutto-wrapped beef in the middle and wrap the dough around the meat, sealing the edges.
Flip the whole thing over so that edges/seams are at the bottom. Brush with a beaten egg yolk.
Bake for 45 minutes in a 425 degree F oven (tent with foil if the pastry is getting too brown after 20 minutes). Allow to rest for 10 minutes before cutting.