Combine onion, olive oil, balsamic vinegar, 0.25 tsp kosher salt, and pepper. Set aside for 5 minutes.
Chop tomatoes and place in a large bowl. Mix with garlic, basil, onion-balsamic blend, and additional 0.125 tsp salt and pepper. Let sit for at least 10 minutes.
Preheat grill to medium-high and oil the grates generously.
Season salmon with salt and pepper. Place skin-side-down on the grill. Cover and cook for 8-10 minutes until just cooked.
Remove salmon, cover with foil, and let rest for 2-3 minutes while preparing the avocado topping.
Top each salmon fillet with 0.5 cup of prepared avocado bruschetta and serve.
Notes
Perfect for a quick, healthy dinner with fresh ingredients.