Slide the sliced red onion into a bowl of ice water and let it sit for 10 minutes; this softens the sharpness. Drain well and pat dry.
In a small jar, combine the olive oil, lime juice, red wine vinegar, grated garlic, kosher salt, black pepper, and cumin. Seal and shake until the dressing looks glossy; set aside 5 minutes to let the flavors meld.
Add the diced tomatoes to a large bowl and sprinkle with a pinch of salt. Let them stand for 5 minutes, then tip out any excess liquid so the salad stays bright and not watery.
Fold the drained onion and chopped cilantro into the tomatoes. Pour in about half the dressing and toss gently to coat. Let the mixture rest for another 5 minutes to lightly marinate.
Just before serving, add the avocado chunks and the remaining dressing. Gently fold until everything is glistening and evenly seasoned. Taste and adjust salt or lime as needed, then serve right away.