2 ounces dried porcini mushrooms
2 cups boiling water
3 tablespoons unsalted butter
3 large shallots, minced
2 garlic cloves, minced
4 scallions, minced
Salt and freshly ground pepper
One 3-pound center-cut trimmed beef tenderloin in 1 piece
3 1/2 tablespoons vegetable oil
7 ounces thinly sliced pancetta, chilled
Preheat the oven to 450 degrees. Soak the porcini in the boiling water for 20 minutes. Rub the porcini in the soaking liquid to remove any grit; pat dry and coarsely chop. Transfer the porcini to a mini food processor and puree.
Melt the butter in a medium skillet. Add the shallots and cook over moderately low heat until softened. Add the garlic and cook for 2 minutes. Add the scallions and cook until softened, 2 minutes. Stir in the porcini puree. Season with salt and pepper. Let cool.
Season the roast with salt and pepper. Heat 2 tablespoons of the oil in a large skillet. Sear the roast over moderately high heat until browned all over; let cool.
On a work surface, slightly overlap three 16-inch pieces of wax paper. Arrange the pancetta on the paper in 4 overlapping rows to form a rectangle the length of the tenderloin. Spread the porcini puree over the pancetta. Set the tenderloin on the bottom edge of the pancetta. Using the wax paper as a guide, tightly roll up the roast in the pancetta. Carefully peel off the wax paper.
Tie the roast with kitchen string at 1-inch intervals. Transfer to a rimmed baking sheet and brush with the remaining 1 1/2 tablespoons of oil. Roast the beef for 25 minutes, or until the pancetta is crisp and an instant-read thermometer inserted in the center of the meat registers 120 degrees.
Let rest for 10 minutes. Cut off the strings and remove. Thickly slice the roast with a serrated knife and serve.