Combine onion, olive oil, balsamic, kosher salt and pepper. Set aside for 5 minutes.
Chop tomatoes and place in a large bowl. Combine with garlic, basil, onion-balsamic mix and additional kosher salt and pepper to taste. Let sit for at least 10 minutes.
Preheat a gas grill to medium-high. Oil the grates generously.
Season salmon with salt and black pepper. Place skin-side down on the grill. Cook for 8 to 10 minutes, or until just cooked through.
Remove salmon and cover with foil; rest for 2 to 3 minutes while adding avocado to the bruschetta.
Serve each piece topped with a generous portion of avocado bruschetta.