Heat olive oil in a skillet over medium-high heat and sear the beef on all sides until browned, about 4 minutes per side. Remove from skillet and let cool.
In the same skillet, add mushrooms and garlic. Cook until mushrooms release their liquid, about 5 minutes.
Add brandy and cook until evaporated, about 2 minutes. Set aside to cool.
Roll out puff pastry on a floured surface to a 1/4-inch thickness. Spread mustard over the beef and top with mushroom mixture.
Wrap the beef in puff pastry, sealing the edges with beaten egg. Place seam side down on a baking sheet.
Brush the top with more beaten egg and bake for 40 minutes or until pastry is golden brown and beef is cooked to desired doneness.
Let rest for 10 minutes before slicing and serving.
