This is commonly called salsa verde due to its green color. Roasting the vegetables gives this version an extra layer of smoky flavor.
Place tomatillos, jalapeno peppers and garlic on foil-lined baking sheet. Broil five minutes on one side; then turn vegetables over and broil another 5-10 minutes until tomatillos turn an olive-green color and jalapeno peppers have some brown spots. Tomatillos will feel slightly soft to the touch. Cool.
Remove stem and seeds from jalapeno peppers; place tomatillos, jalapeno peppers and garlic in blender; blend until you have a coarse puree. Add cilantro and water and continue blending briefly until cilantro is finely chopped. Pour into serving dish and add minced onion; season to taste with salt.
Adapted from Rick Bayless.