Mummified Beef Wellington

Name: Mummified Beef Wellington
Ingredients: One 1 1/2- to 1 3/4-pound beef tenderloin, tail end; Kosher salt and freshly ground black pepper; 2 tablespoons olive oil; 2 pints cremini mushrooms; 2 cloves garlic, chopped; 1 shallot, roughly chopped; 4 tablespoons unsalted butter; 1/4 cup dry sherry; 1 tablespoon fresh thyme, minced; 4 slices prosciutto; 1 sheet frozen puff pastry, thawed; All-purpose flour, for dusting; 1 large green olive stuffed with pimentos, sliced, for the eyes; Nonstick cooking spray, for the baking sheet; 1 large egg beaten with 1 teaspoon water, for the egg wash; 3 tablespoons unsalted butter, at room temperature; 1 medium shallot, minced; 1 clove garlic, minced; 1 1/2 cups red wine; 2 cups beef stock; 1 tablespoon all-purpose flour