Preheat a medium cast iron skillet or other heavy bottomed frying pan over high heat. Rub beef with olive oil and seson with salt and pepper. Place beef in hot pan and cook, turning once or twice until crusty brown outside but still quite rare inside, 4-5 minutes total. Remove to a plate and let cool. Cover and refrigerate until well chilled, about 4 hours or as long as 2 days.
On a lightly floured work surface, roll each pastry sheet to a thickness of about 1/8 inch. Using an oval biscuit cutter preferably with a crinkled edge make a total of 96 (2 1/4 inch) pastry ovals. Arrange half of them 1 inch apart on foil or parchment lined baking sheets and brush lightly with water. Using a small round biscuit cutter, remove a 3/4-1 inch circle from center of remaining pastry ovals. Gently place each hollow oval on top of moistened pastry. aligning edges to form a sandwich. cover and refrigerate until thoroughly chilled 30 minutes or as long as 2 days. Preheat oven to 400 degrees.
Bake chilled pastry until puffed and golden brown 15 to 20 minutes. transfer to a rack and let cool. If any of the center indentations have puffed up, press down with a wooden spoon handle (if made in advance. store airtight at room temperature up to 2 days. Freeze for longer storage)
Remove any visible fat from chilled beef and cut into 1/4 to 1/2 inch chunks to fit inside pastries. Top with about 1/2 teaspoon mushroom duxelles. Bake until pastry is crisp and mushroom topping bubbly hot, 5-8 minutes. Garnish with a watercress leaf. Serve warm.