Place tomatillos, onion, jalapenos, and garlic cloves on a baking sheet. Roast for about 20 minutes until the tomatillos are soft and slightly charred. If veggies are soft but not charred you can place under broiler for 2-4 minutes. Watch closely and remove once desired charring is achieved.
1 lb tomatillos, husked and rinsed, 1 medium onion, quartered, 2 jalapeno, stemmed (seeds removed for less heat), 2 cloves garlic