3 pints (1 1/2 pounds) white button mushrooms
2 shallots, peeled and roughly chopped
4 cloves garlic, peeled and roughly chopped
2 sprigs fresh thyme, leaves only
2 tablespoons unsalted butter
2 tablespoons extra-virgin olive oil
Kosher salt and freshly ground black pepper
1 (3-pound) center cut beef tenderloin (filet mignon), trimmed
Extra-virgin olive oil
Kosher salt and freshly ground black pepper
12 thin slices prosciutto
6 sprigs of fresh thyme, leaves only
2 tablespoons Dijon mustard
Flour, for rolling out puff pastry
1 pound puff pastry, thawed if using frozen
2 large eggs, lightly beaten
1/2 teaspoon coarse sea salt
Minced chives for garnish