Pull off and discard tomatillo husks. Rinse and dry tomatillos.
In a 10- to 12-inch frying pan over medium-high heat, cook tomatillos and garlic until lightly browned, about 5 minutes, turning often. Remove from pan to cool.
Peel garlic and mash to a paste with 1/4 teaspoon salt. Mix with oil and vinegar.
Rinse fish and pat dry. Coat skinless sides of salmon with garlic mixture. Let stand about 5 minutes.
For minimum heat, remove seeds and veins from chilies. Coarsely chop chilies.
In a food processor or blender, combine chilies, tomatillos, 1/4 cup cilantro, and lime juice. Whirl until puréed.
Peel, pit, and coarsely dice avocado. Add half the avocado to tomatillo mixture and whirl just to blend slightly. Pour into a bowl and stir in onion, remaining diced avocado, and salt to taste.
Lay fish on an oiled grill over a solid bed of hot coals or high heat on a gas grill (you can hold your hand at grill level only 2 to 3 seconds); close lid on gas grill. Cook salmon, turning once, until opaque but still moist-looking in center of thickest part (cut to test), 6 to 8 minutes.
Transfer fish to plates, garnish with remaining cilantro, and top with tomatillo-avocado salsa. Add salt to taste.