Prepare the Onions:

Place the thinly sliced red onions in a bowl of ice-cold water and let them soak for 10-15 minutes. This process helps to mellow the sharpness of the onions.

Drain and Combine:

Drain the onions thoroughly and transfer them to a large mixing bowl.

Add the diced tomatoes, chopped cilantro, and sliced jalapeños (if using) to the bowl with the onions.

Dress the Salad:

In a small bowl, whisk together the fresh lime juice and sea salt until the salt is dissolved.

Pour the lime dressing over the onion mixture and toss gently to combine, ensuring all ingredients are well coated.

Optional Additions:

Gently fold in the diced avocado if desired, taking care not to mash the pieces.

Marinate:

Allow the salad to sit at room temperature for about 10-15 minutes to let the flavors meld together.

Serve:

Give the salad a final toss and adjust seasoning with additional salt or lime juice if needed.

Serve immediately as a side dish or as a topping for tacos, grilled meats, or other Mexican dishes.