2 tablespoons unsalted butter
1 pound button mushrooms, finely chopped
1 teaspoon fresh thyme leaves (from about 2 sprigs)
1 tablespoon sherry vinegar
1 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
Two 8-ounce beef filets, each cut into 8 cubes (16 cubes total)
1 tablespoon olive oil
2 tablespoons Dijon mustard
8 thin slices prosciutto, halved crosswise
1 sheet frozen puff pastry, thawed
1 large egg yolk, lightly beaten
Sea salt, for sprinkling