Preheat BBQ grill to 450°F. or preheat grill pan on stove over medium-high heat.
In a bowl, whisk together the oil, lime juice, salt and pepper. Add the diced avocado, mangoes, onion, serrano chili and cilantro. Toss to coat and set aside.
Rub olive oil or cooking spray on both sides of salmon filets and sprinkle with kosher salt and black pepper.
Place salmon skin side down first and grill 3 minutes, then flip fish over and cook for another 3 minutes. Salmon should look opaque and flake easily when pulled with tines of a fork. Remove from grill and let fish rest a minute or two before serving.
Serve grilled salmon topped with mango avocado salsa and lime slices (optional).