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 food you can replace jalapeños with diced bell pepper. If you'd like it spicier you can leave the ribs and seeds of the jalapeños or even use a hotter pepper like one serrano or habanero pepper.
This salsa is best served within a few hours of preparing as the avocado may start to brown. If you'd like to prepare it in advance just toss everything together except the avocado then store in the fridge for a few days, then add the avocados when ready to serve.