Beef Wellington for Christmas Dinner, Thursday, Dec. 5, 2019. Photo by Hillary Levin, [email protected] Hillary Levin, Post-Dispatch 3 pints (1½ pounds) white button mushrooms 2 shallots, peeled and roughly chopped 8 sprigs fresh thyme, leaves only, divided 1 (3-pound) center-cut beef tenderloin, trimmed 12 thin slices prosciutto 2 tablespoons Dijon or English mustard Flour, for rolling out puff pastry 1 pound puff pastry, homemade (see recipe) or store-bought, thawed if frozen. 2 large eggs, lightly beaten ½ teaspoon coarse sea salt Note: The duxelles and the homemade puff pastry (if using) can be made a day or two ahead of