Place tomatillos in a saucepan and cover with water. Bring to a boil, then simmer for 5 minutes. Remove tomatillos and when cool enough to handle, roughly chop them.
In a non-reactive bowl, combine the chopped tomatillos, onion, cilantro and lime juice. Gently fold in diced avocado and jalapeño. Season with salt to taste. Refrigerate until ready to use.
Preheat oven to 350 degrees.
In a small bowl, combine chili powder, cumin, sugar and salt. Sprinkle fish with chili rub.
Heat a large oven-proof nonstick skillet over medium heat. Cook the salmon, rub side down for 2 minutes, then flip and place pan in the oven for about 6-8 minutes or until cooked through. Transfer fish fillets to plates and top with the Avocado Tomatillo Salsa.