1. Combine onion, olive oil, balsamic, 1/4 tsp kosher salt and pepper. Set aside a 5 minutes.
2. Chop tomatoes and place in a large bowl.
3. Combine with garlic, basil, onion-balsamic combo and additional 1/8 tsp salt and pepper to taste. Set aside and let it sit at least 10 minutes.
4. Preheat a gas grill to medium-high. Oil the grates generously to prevent the fish from sticking.
5. Season salmon with salt and fresh ground black pepper to taste.
6. Place skin-side-down on the grill. Close the lid and cook without turning for 8 o 10 minutes (depending on thickness), or until the salmon is just cooked through. The skin will char but will prevent the fish from drying out.
7. Remove and cover with foil; rest for 2 to 3 minutes while adding the avocado to the bruschetta.
8. Serve topped each with generous 1/2 cup avocado bruschetta.