Slice the red onions very thinly using a sharp knife or a mandoline slicer.
Sprinkle the sliced onions with 1 tablespoon salt and rub the salt into the onions.
Cover the onions with cold water and rinse well until the salt is removed. If the onions are very strong then you can let them rest in the water for 5-10 minutes.
Drain the onions and place them in a bowl.
Add the lime juice and a sprinkle of salt, let rest until the onions start to turn pinkish, about 5-10 minutes.
The tomatoes can be sliced thinly or diced. Add the tomatoes to the onion curtido mix.
Add the oil and the chopped cilantro. Mix well, taste and add additional salt to taste if needed.
Notes
Curtido variations can also include thinly sliced carrots, bell peppers, cucumbers, radishes, etc.
To turn this curtido into a complete salad, serve over chopped lettuce or salad greens and add avocado slices.