In a medium serving bowl, combine the lemon juice, minced red onion, minced jalapeño, and turmeric, if using.
Sprinkle with salt and pepper and stir to combine.
Add the chopped tomatoes to the bowl and toss gently to coat everything in the zesty dressing.
Chill for at least 15 minutes (and up to 30) to allow the flavors to meld.
Taste and adjust seasoning before serving.
Don't skip the chilling time! This step is crucial for the onion and jalapeño to marinate in the lemon juice and for all the flavors to meld together.