Preheat oven to the temperature on the puff pastry cup box.
Cut steak into 1-inch thick pieces. Season both sides with salt and pepper.
Heat oil over a skillet over high heat. Sear each piece of meat on each side for about 45 seconds - 1 minute.
After taking the meat from the skillet, immediately brush every side lightly with dijon mustard.
Cut each piece of proscuitto into three long slices. Cover with a thin layer of duxelle.
Place one piece of steak at the end of one of the proscuito and roll it up. Repeat until all of the pieces of steak are rolled up.
Place the puff pastry cups on a baking sheet, and bake for 10 minutes.
After ten minutes, gently place one piece of steak on top of each puff pastry cup. Continue to bake for another ten minutes.
In a food processor, place mushrooms, onions, and garlic and pulse until a slightly pasty mixture is created.
In a sauce pan, heat butter until melted.
Add the paste, chicken brother, and parsley to the melted butter, and cook over medium heat until the liquid cooks out (about 4-5 minutes)
Remove from heat and cool completely.