Preheat your broiler on high and place a rack as near the top of yur oven that you can. Husk and core your tomatillos, then place them, along with the serrano peppers on a broiler pan. Roast in the oven until black, then flip and roast the other side until black.
Place the garlic in small bowl with just enough water to cover. Microwave on high for one minute. Drain.
Remove stems and seeds (optional on the seeds) from the serrano peppers, then put everything in your blender and pulse until smooth. If necessary, thin with water.