1. Season beef with salt and pepper.
2. Melt the butter in the Casserole Pan on medium heat.
Sauté the shallot and mushrooms until all liquid has reduced. Remove from the pan and set aside to cool.
3. Place the Casserole Pan on high heat. Add olive oil & sear the beef on all sides. Set aside until cooled.
4. Beat the egg yolk and milk. Preheat the oven to 375° F.
5. Place the puff pastry on the counter. Place the ham onto the pastry. Top with shallots and mushrooms.
6. Brush the beef with the mustard. Arrange carefully on top of ham and mushrooms. Brush the edges of the pastry with egg mixture.
7. Roll into a Wellington & brush the outside with the rest of the egg mixture.
8. Score the top of the pastry with a knife. Bake in the oven until golden.