Step by Step method

1. Step 1.

   Toast garlic in large skillet over medium-low heat, shaking pan occasionally, until fragrant and spotty brown, about 5 minutes. When cool enough to handle, peel garlic and transfer to blender with parsley, scallions, vinegar, and pepper flakes. With blender running, slowly drizzle in 4 tablespoons oil until smooth and emulsified. Season with salt and pepper.

2. Step 2.

   Pat steak dry with paper towels and season with salt and pepper. Heat remaining oil in empty skillet over medium-high heat until just smoking. Cook steak until well browned, 5 to 7 minutes per side. Transfer to cutting board, tent with foil, and let rest 5 minutes. Slice steak thinly against grain. Serve, passing sauce at table.