Combine chopped red onion, olive oil, balsamic vinegar, salt, and pepper. Set aside for 5 minutes.

Chop tomatoes and place in a large bowl. Add minced garlic, basil, the onion-balsamic mixture, and season with salt and pepper to taste. Let sit for at least 10 minutes.

Preheat grill to medium-high and oil the grates to prevent sticking.

Season salmon fillets with salt and pepper. Place skin-side down on the grill, close lid, and cook for 8-10 minutes until just cooked through.

Remove salmon and let rest for 2-3 minutes. Flake salmon and serve topped with the fresh tomato and basil mixture and diced avocado.