1. Rinse the rice under cold running water until the water runs clear. Drain thoroughly.
2. Place the rice and water in a heavy saucepan with a tight-fitting lid. Bring to a boil over high heat, stirring gently to keep the rice from sticking to the bottom of the pan. When the water is boiling rapidly, stop stirring, cover the pan with the lid and turn the heat down to low. Let the rice cook, without lifting the lid, for 20 minutes.
4. Remove from the heat, lift the lid to release the steam, and replace it immediately. Let stand, covered, for 10 minutes.
5. While the rice is cooking, rinse and seed the ancho chilies. Put the onion and all of the chilies in a saucepan with the oil. Cook over medium heat to toast the chilies, about 2 to 3 minutes.
6. Take the chilies out and add the tomatillos, water and garlic and cook for 15 minutes, stirring constantly.
7. Place the cooked ingredients, plus the chilies, cilantro and cumin into a blender and pulse for 10 seconds or until chunky.
8. Stir the salsa into the cooked rice. Season to taste.
For the shrimp:
1. Heat a large skillet or wok over medium-high heat. Add the garlic and peanut oil and stir quickly for 30 seconds.
2. Raise the heat to high and add the shrimp, stirring quickly for about another minute. Season with pepper and cook until the shrimp are pink and opaque, about 2 minutes.
3. Place the shrimp on top of the green rice.