Place red onion in a strainer or sieve and rinse under cool water to remove harsh bite. Drain well. Add to a mixing bowl along with diced tomatoes, jalapeños and avocados.
In a separate small mixing bowl whisk together olive oil, lime juice, garlic, salt and pepper until mixture is well blended. Pour mixture over avocado mixture, add cilantro then gently toss mixture to evenly coat. Serve with tortilla chips or over Mexican entrees.
Notes
If you aren't a fan of spicy foods, you can definitely omit the jalapeños. And if you like it spicier you can leave in the seeds or even use hotter peppers like serrano or habanero.