Use a clean cutting board to prepare the dressing.
Place peeled garlic clove in a small microwave-safe bowl with 1 tablespoon of water. Microwave for 45 seconds to 1 minute. Garlic should be a little soft, but still firm. This will reduce the acidity of the raw garlic, but retain much of its flavor. Let stand for 15 seconds.
Chop green onion (white and green portions), flat-leaf parsley, garlic, and serrano pepper right on the cutting board. Add olive oil and vinegar. Continue chopping; basically, fine dicing, while incorporating the liquids. Season with salt and pepper and mix through. If dressing is too dry, add another tablespoon of olive oil to the dressing.
Slice the cooked steak, and add pieces straight onto the cutting board. Using tongs, gently toss the meat with board dressing. Serve immediately.