Combine garlic, soy sauce, vinegar, oil, brown sugar and ginger in a blender or food processor; blend until smooth. Set aside.
Press peppercorns into both sides of sirloin. Season with salt. Place the sirloin, scallions and bell pepper halves on the grill and cook for 4 minutes. Turn over and cook until the meat is medium-rare and the vegetables are slightly charred, about 4 minutes more.
Let sirloin rest for 5 minutes, then slice thinly, against the grain. Cut scallions into 1-inch pieces. Slice the pepper lengthwise into strips.
Toss greens with the reserved dressing in a large bowl. Arrange on a platter or 4 plates. Top with sirloin and grilled vegetables. Serve immediately.