1. Preheat the oven to 450°. 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.
2. 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.
3. 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.
4. On a work suface, slightly overlap 3 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.
5. 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°. Let rest for 10 minutes. Cut off the strings and remove. Thickly slice the roast with a serrated knife and serve.