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.

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.