Peel the outer skins/husks off the tomatillos, onion and garlic. Cut the stems off the peppers and slice in half lengthwise. Cut the onion in half. (See Note 1)
In the Dutch oven or large stock pot bring 6 cups of water to a boil. Add the tomatillos, chiles and onion. Cook for 8 minutes.
Carefully remove with a slotted spoon and place all the peppers, tomatillos, onion, garlic and cilantro in a food processor, blender or Vitamix, and process until smooth. Use 2 cups of the salsa for this recipe and reserve remaining for other use.
In a bowl season the shrimp with kosher salt, black pepper and garlic powder.
In a large skillet add oil and sautée shrimp 2 minutes per side. Pour the 2 cups of salsa verde on top and cook 2 minutes more, stirring. Stir in half of the cilantro leaves.
Spoon over steamed rice and top with remaining cilantro.
1. If you prefer, you can either grill or place the tomatillos, chiles and onion under the broiler for a charred flavor instead of boiling, then proceed with puréeing.