Beef Wellington

Name: Beef Wellington
Prep time: 15 min
Total time: 15 min
Cuisine: European, British
Ingredients: 1 (2 lb.) center-cut beef tenderloin, trimmed; kosher salt; Freshly ground black pepper; Olive oil, for greasing; 2 tbsp. Dijon mustard; 1 1/2 lb. mixed mushrooms, roughly chopped; 1 shallot, roughly chopped; Leaves from 1 thyme sprig; 2 tbsp. unsalted butter; 12 thin slices prosciutto; Flour, for dusting; 14 oz. frozen puff pastry, thawed; 1 large egg, beaten; Flaky salt, for sprinkling