"Trust us, it's worth the effort...."
(2 lb.) center-cut beef tenderloin, trimmed
kosher salt
Freshly ground black pepper
Olive oil, for greasing
Dijon mustard
mixed mushrooms, roughly chopped
shallot, roughly chopped
Leaves from 1 thyme sprig
unsalted butter
thin slices prosciutto
Flour, for dusting
frozen puff pastry, thawed
large egg, beaten
Flaky salt, for sprinkling