Bring a small saucepan of salted water to a boil; add onions and immediately remove saucepan from heat. Let onions blanche in hot water until softened, 1 to 2 minutes. Drain and rinse onions with cold water.
Combine cilantro and lime juice in a sealable plastic bag, squeeze bag to remove as much air as possible from the bag before sealing, and massage the lime juice into the cilantro leaves through the bag until the leaves are all moistened.
Turn the cilantro out onto a cutting board and chop; transfer to a bowl. Stir onion, tomatoes, jalapeno pepper, garlic, salt, and pepper with the cilantro mixture.
Cover bowl with plastic wrap and refrigerate for 1 hour.