How to Make It
Preheat oven to 400°F. Rub beef all over with 1 tablespoon pepper and 2 teaspoons salt. Heat 1 tablespoon olive oil in a large cast-iron skillet over medium-high until shimmering. Add beef, and cook, turning occasionally, until browned all over, 6 to 8 minutes. Transfer skillet to preheated oven, and roast until an instant-read thermometer inserted in thickest portion registers 120°F, 12 to 15 minutes. Transfer to a carving board, and let rest 20 minutes.
Meanwhile, whisk together garlic and remaining 1/4 cup olive oil in a small bowl. Arrange mushrooms in an even layer on a large rimmed baking sheet, and spoon garlic oil evenly over mushrooms. Sprinkle with remaining 1 teaspoon pepper and remaining 1 teaspoon salt. Bake at 400°F until lightly browned and just tender, 12 to 15 minutes. Let mixture cool.
Cut off and discard twine around beef. Cut beef into 16 thin slices; cut slices in half. Spoon mushroom mixture evenly onto center of beef pieces; gather edges, and secure each with a cocktail pick, forming little bundles. Transfer bundles to a platter or board. Garnish with parsley, and serve warm or at room temperature.